Compact holographic display device

ABSTRACT

A holographic display device comprises an OLED array writing onto an OASLM, the OLED array and OASLM forming adjacent layers. The OASLM encodes a hologram and a holographic reconstruction is then generated by the device when an array of read beams illuminates the OASLM. The OASLM is suitably controlled by the OLED array. An advantage of the device is that it lends itself to compactness.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of PCT/EP2007/061504, filed on Oct.26, 2007, which claims priority to Great Britain (GB) Application No.0621360.7, filed Oct. 26, 2006; GB Application No. 0625838.8, filed Dec.22, 2006; GB Application No. 0705412.5, filed Mar. 21, 2007; GBApplication No. 0705410.9, filed Mar. 21, 2007; GB Application No.0705408.3, filed Mar. 21, 2007; GB Application No. 0705407.5, filed Mar.21, 2007; GB Application No. 0705403.4, filed Mar. 21, 2007; GBApplication No. 0705401.8, filed Mar. 21, 2007; GB Application No.0705399.4, filed Mar. 21, 2007; GB Application No. 0705398.6, filed Mar.21, 2007; GB Application No. 0705409.1, filed Mar. 21, 2007; GBApplication No. 0705411.7, filed Mar. 21, 2007; GB Application No.0705406.7, filed Mar. 21, 2007; GB 0705402.6, filed Mar. 21, 2007; GBApplication No. 0705405.9, filed Mar. 21, 2007; and GB Application No.0705404.2, filed Mar. 21, 2007, the entire contents of which are herebyincorporated in total by reference.

The present application is related to the following applications alsofiled contemporaneously herewith: Ser. No. 12/447,111 entitled COMPACTHOLOGRAPHIC DISPLAY DEVICE, Ser. No. 12/447,116 entitled COMPACTHOLOGRAPHIC DISPLAY DEVICE, Ser. No. 12/447,125 entitled MOBILETELEPHONY SYSTEM COMPRISING HOLOGRAPHIC DISPLAY, Ser. No. 12/447,133entitled 3D CONTENT GENERATION SYSTEM, Ser. No. 12/447,144 entitledHOLOGRAPHIC DISPLAY DEVICE, and Ser. No. 12/447,149 entitled HOLOGRAPHICDISPLAY DEVICE COMPRISING MAGNETO-OPTICAL SPATIAL LIGHT MODULATOR.

The above are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device for generating three dimensionalimages, especially where the device is a compact device including adisplay on which computer-generated video holograms (CGHs) are encodedon one or two optically addressable spatial light modulators. Thedisplay generates three dimensional holographic reconstructions. Thedevice has particular application in portable devices and in handhelddevices, such as mobile telephones.

2. Technical Background

Computer-generated video holograms (CGHs) are encoded in one or morespatial light modulators (SLMs); the SLMs may include electrically oroptically controllable cells. The cells modulate the amplitude and/orphase of light by encoding hologram values corresponding to avideo-hologram. The CGH may be calculated e.g. by coherent ray tracing,by simulating the interference between light reflected by the scene anda reference wave, or by Fourier or Fresnel transforms. An ideal SLMwould be capable of representing arbitrary complex-valued numbers, i.e.of separately controlling the amplitude and the phase of an incominglight wave. However, a typical SLM controls only one property, eitheramplitude or phase, with the undesirable side effect of also affectingthe other property. There are different ways to modulate the light inamplitude or phase, e.g. electrically addressed liquid crystal SLM,optically addressed liquid crystal SLM, magneto-optical SLM, micromirror devices or acousto-optic modulators. The modulation of the lightmay be spatially continuous or composed of individually addressablecells, one-dimensionally or two-dimensionally arranged, binary,multi-level or continuous.

In the present document, the term “encoding” denotes the way in whichregions of a spatial light modulator are supplied with control values toencode a hologram so that a 3D-scene can be reconstructed from the SLM.By “SLM encoding a hologram” it is meant that a hologram is encoded onthe SLM.

In contrast to purely auto-stereoscopic displays, with video hologramsan observer sees an optical reconstruction of a light wave front of athree-dimensional scene. The 3D-scene is reconstructed in a space thatstretches between the eyes of an observer and the spatial lightmodulator (SLM), or possibly even behind the SLM. The SLM can also beencoded with video holograms such that the observer sees objects of areconstructed three-dimensional scene in front of the SLM and otherobjects on or behind the SLM.

The cells of the spatial light modulator are preferably transmissivecells which are passed through by light, the rays of which are capableof generating interference at least at a defined position and over aspatial coherence length of a few millimetres. This allows holographicreconstruction with an adequate resolution in at least one dimension.This kind of light will be referred to as ‘sufficiently coherent light’.

In order to ensure sufficient temporal coherence, the spectrum of thelight emitted by the light source must be limited to an adequatelynarrow wavelength range, i.e. it must be near-monochromatic. Thespectral bandwidth of high-brightness LEDs is sufficiently narrow toensure temporal coherence for holographic reconstruction. Thediffraction angle at the SLM is proportional to the wavelength, whichmeans that only a monochromatic source will lead to a sharpreconstruction of object points. A broadened spectrum will lead tobroadened object points and smeared object reconstructions. The spectrumof a laser source can be regarded as monochromatic. The spectral linewidth of a LED is sufficiently narrow to facilitate goodreconstructions.

Spatial coherence relates to the lateral extent of the light source.Conventional light sources, like LEDs or Cold Cathode Fluorescent Lamps(CCFLs), can also meet these requirements if they radiate light throughan adequately narrow aperture. Light from a laser source can be regardedas emanating from a point source within diffraction limits and,depending on the modal purity, leads to a sharp reconstruction of theobject, i.e. each object point is reconstructed as a point withindiffraction limits.

Light from a spatially incoherent source is laterally extended andcauses a smearing of the reconstructed object. The amount of smearing isgiven by the broadened size of an object point reconstructed at a givenposition. In order to use a spatially incoherent source for hologramreconstruction, a trade-off has to be found between brightness andlimiting the lateral extent of the source with an aperture. The smallerthe light source, the better is its spatial coherence.

A line light source can be considered to be a point light source if seenfrom a right angle to its longitudinal extension. Light waves can thuspropagate coherently in that direction, but incoherently in all otherdirections.

In general, a hologram reconstructs a scene holographically by coherentsuperposition of waves in the horizontal and the vertical directions.Such a video hologram is called a full-parallax hologram. Thereconstructed object can be viewed with motion parallax in thehorizontal and the vertical directions, like a real object. However, alarge viewing angle requires high resolution in both the horizontal andthe vertical direction of the SLM.

Often, the requirements on the SLM are lessened by restriction to ahorizontal-parallax-only (HPO) hologram. The holographic reconstructiontakes place only in the horizontal direction, whereas there is noholographic reconstruction in the vertical direction. This results in areconstructed object with horizontal motion parallax. The perspectiveview does not change upon vertical motion. A HPO hologram requires lessresolution of the SLM in the vertical direction than a full-parallaxhologram. A vertical-parallax-only (VPO) hologram is also possible butuncommon. The holographic reconstruction occurs only in the verticaldirection and results in a reconstructed object with vertical motionparallax. There is no motion parallax in the horizontal direction. Thedifferent perspective views for the left eye and right eye have to becreated separately.

3. Discussion of Related Art

Typically, devices for generating three dimensional images have lackedcompactness—i.e. they require complex and bulky optical systems thatpreclude their use in portable devices, or in handheld devices, such asmobile telephones. U.S. Pat. No. 4,208,086 for example describes adevice for generating large three dimensional images, where the deviceis of the order of a metre in length. WO 2004/044659 (US2006/0055994),which is incorporated herein by reference, describes a device forreconstructing video three dimensional images with a depth in excess often centimetres. Such prior art devices are therefore too deep formobile phones or other portable or handheld, small display devices.

WO 2004/044659 (US2006/0055994) filed by the applicant describes adevice for reconstructing three-dimensional scenes by way of diffractionof sufficiently coherent light; the device includes a point light sourceor line light source, a lens for focusing the light and a spatial lightmodulator. In contrast to conventional holographic displays, the SLM intransmission mode reconstructs a 3D-scene in at least one ‘virtualobserver window’ (see Appendix I and II for a discussion of this termand the related technology). Each virtual observer window is situatednear the observer's eyes and is restricted in size so that the virtualobserver windows are situated in a single diffraction order, so thateach eye sees the complete reconstruction of the three-dimensional scenein a frustum-shaped reconstruction space, which stretches between theSLM surface and the virtual observer window. To allow a holographicreconstruction free of disturbance, the virtual observer window sizemust not exceed the periodicity interval of one diffraction order of thereconstruction. However, it must be at least large enough to enable aviewer to see the entire reconstruction of the 3D-scene through thewindow(s). The other eye can see through the same virtual observerwindow, or is assigned a second virtual observer window, which isaccordingly created by a second light source. Here, a visibility region,which would typically be rather large, is limited to the locallypositioned virtual observer windows. The known solution reconstructs ina diminutive fashion the large area resulting from a high resolution ofa conventional SLM surface, reducing it to the size of the virtualobserver windows. This leads to the effect that the diffraction angles,which are small due to geometrical reasons, and the resolution ofcurrent generation SLMs are sufficient to achieve a high-qualityreal-time holographic reconstruction using reasonable, consumer levelcomputing equipment.

However, the known method of generating a three dimensional imageexhibits the disadvantage that a large, voluminous, heavy and thusexpensive lens is required for focusing due to the large SLM surfacearea. Consequently, the device wilt have a large depth and weight.Another disadvantage is represented by the fact that the reconstructionquality is reduced significantly due to aberrations at the margins (i.e.the edges) when using such large lenses. An improvement in which a lightsource including a lenticular array is used is disclosed in US2006/250671, which is incorporated herein by reference, although thedisclosure is for the case of large area video holograms.

A mobile phone which generates a three dimensional image is disclosed inUS2004/0223049. However, the three dimensional image disclosed thereinis generated using autostereoscopy. One problem withautostereoscopically generated three dimensional images is thattypically the viewer perceives the image to be inside the display,whereas the viewer's eyes tend to focus on the surface of the display.This disparity between where the viewer's eyes focus and the perceivedposition of the three dimensional image leads to viewer discomfort aftersome time in many cases. This problem does not occur, or issignificantly reduced, in the case of three dimensional images generatedby holography.

SUMMARY OF THE INVENTION

In a first aspect, a holographic display device is provided comprisingan OLED array writing onto an OASLM, the OLED array and OASLM formingadjacent layers, the OASLM encoding a hologram and a holographicreconstruction being generated by the device when an array of read beamsilluminates the OASLM and the OASLM is suitably controlled by the OLEDarray. The OLED array and OASLM may form facing, adjacent layers with nointermediary imaging optics between the OLED array and OASLM. The OLEDarray and OASLM may be in fixed, direct physical attachment with oneanother, or the OLED array and OASLM may be in fixed, indirect physicalattachment with one another. The OLED array and OASLM may be physicallyattached to one another indirectly via an isolation layer. The isolationlayer may be an angular filter such as a Bragg filter.

In one implementation, there is provided an array of infra red emittingorganic light emitting diodes (OLED) on a substrate; the substrate istransparent to visible light, and the array of infra red emittingorganic light emitting diodes is in close proximity to an opticallyaddressable spatial light modulator (OASLM). The infra red light permitscontrol of the amplitude or phase, or some combination of the amplitudeand phase, of the visible light transmitted by the OASLM.

Being present as an array, the infra red emitting OLEDs permit controlof the spatial distribution of the amplitude or phase, or somecombination of the amplitude and phase, of the visible light transmittedby the OASLM. The OLED array and OASLM are located in close proximitysuch that they form a compact pair. The compact OLED array and OASLMpair act on visible light such as to generate a hologram in the OASLM. Athree dimensional image may then be viewed by a viewer located at somedistance from the compact OLED array and OASLM pair.

The OLED array may emit a non-primary colour display wavelength and theread-out wavelengths may be one or more of RGB. The OLED array may be IRemitting and it may write to an IR sensitive layer on the OASLM. TheOLED array and OASLM layers may be reflective and visible light may bereflected from the OLED array and OASLM layers to an observer. The OLEDarray may be made up of multiple, smaller tiled OLEDs. The OASLM maycontain liquid crystal material. The OASLM may include a photosensitivedye which acts as a photosensor layer.

The display may be illuminated with a backlight and micro-lens array.The micro-lens array may provide localised coherence over a small regionof the display, that region being the only part of the display thatencodes information used in reconstructing a given point of thereconstructed object. The display may contain a reflective polarizer.The display may contain a prismatic optical film.

The OASLM may be a Freedericksz cell arrangement to give phase control.The holographic reconstruction may be visible through a virtual observerwindow. The virtual observer windows may be tiled using spatial or timemultiplexing. The display may be operable to time sequentially re-encodea hologram on the hologram-bearing medium for the left and then theright eye of an observer.

The display may generate a holographic reconstruction for a single userto view.

The display may have light emitting diodes as its light sources.

The display may generate a 2D image that is in focus on a screenindependent of the distance of the screen from the device in the opticalfar field without the need for any projection lenses.

The display device may be such that a holographic image is sent to eacheye using a beam splitter.

The OASLM may be positioned within 30 mm of a light source and housedwithin a portable casing.

The display device may be such that a beam steering element is presentfor tracking VOWs, the beam steering element consisting of liquidcrystal domains inside an isotropic host material, where the interfacesbetween the domains and the matrix are prism-shaped, or the shape ofsections of a sphere, or the shape of sections of a cylinder, and theorientation of the liquid crystals are controlled using externallyapplied electric fields so as to vary the local refractive ordiffractive properties of the beam steering element.

The display device may be such that the OASLM, a light source and a lensarray aligned with the light source, are all housed within a portablecasing and in which the light source is magnified between 10 and 60times by the lens array.

The OLED array and OASLM layers may be transparent and read-out visiblelight may pass through the layers to an observer.

The OASLM may be sensitive to the write wavelength emitted by the OLEDarray but may not be sensitive to a read-out wavelength.

The OLED array may be yellow emitting and the read-out wavelength may beone or more of RGB.

The OASLM may be continuous.

The OASLM may be made up of multiple, smaller tiled OASLMs.

The display device may encode a hologram and enable a holographicreconstruction to be generated.

The display may be operable such that only when an observer's eyes arepositioned approximately at the image plane of the light source can theholographic reconstruction be seen properly.

The display device may be such that the size of the reconstructed threedimensional scene is a function of the size of the hologram-bearingmedium and the reconstructed three dimensional scene can be anywherewithin a volume defined by the hologram-bearing medium and a virtualobserver window through which the reconstructed three dimensional scenemust be viewed.

The display device may be such that the display encodes a hologramcomprising a region with information needed to reconstruct a singlepoint of a three dimensional scene, the point being visible from adefined viewing position, and: the region (a) encodes information forthat single point in the reconstructed scene and (b) is the only regionin the hologram encoded with information for that point, and (c) isrestricted in size to form a portion of the entire hologram, the sizebeing such that multiple reconstructions of that point caused by higherdiffraction orders are not visible at the defined viewing position.

The display may encode a hologram generated by determining thewavefronts at the approximate observer eye position that would begenerated by a real version of an object to be reconstructed.

The display may be operable such that the holographic reconstruction isthe Fresnel transform of the hologram and not the Fourier transform ofthe hologram.

In a further aspect, a holographic display device is provided comprisingan OLED array writing onto a pair of OASLMs, the OLED array and OASLMsforming adjacent layers, the pair of OASLMs encoding a hologram and aholographic reconstruction being generated by the device when a array ofread beams illuminates the pair of OASLMs and the pair of OASLMs issuitably controlled by the OLED array. The OLED array may emit at 2different wavelengths where one wavelength is used to write/control oneOASLM to modulate phase, and the other is used to write/control theother OASLM to modulate amplitude. The OLED array may be made up of twotypes of OLED which each emit at a different wavelength. Temporalmultiplexing between two emission wavelengths of the OLED array may beused to enable independent control of the two OASLMs.

In a further aspect, a method is provided which consists of generating aholographic reconstruction comprising the step of using a display deviceas described herein.

In a further aspect, a method is provided of manufacturing a displaydevice, including the steps of taking a glass substrate and successivelyprinting or otherwise creating an OLED array and then the layers for anOASLM on the substrate. The method may be such that an isolation layerbetween the OLED and OASLM is a sputtered coating or other coating witha thickness of under 10 microns. The method may be such that theprinting or creation of both the OLED array and OASLM layers areseparate steps in a single fabrication process.

By “SLM encoding a hologram” it is meant that a hologram is encoded onthe SLM.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a holographic display device including a singleOASLM and a single OLED array.

FIG. 2 is a diagram of a holographic display device including a pair ofcomponents, where each component contains a single OASLM and a singleOLED array.

FIG. 3 is a diagram of a mobile three dimensional display device.

FIG. 4 is a diagram of a holographic display according to the prior art.

FIG. 5 is a diagram of a holographic display in which a single array ofOLEDs controls the two OASLMs.

FIG. 6A is a diagram of a holographic display.

FIG. 6B is a diagram of a holographic display which lends itself toachieving compactness.

FIG. 7 is a diagram of a component of a holographic display whichincorporates a Bragg filtering holographic optical element to reduceproblems associated with higher diffraction orders.

FIG. 8 is a diagram of a component of a holographic display whichincorporates a Bragg filtering holographic optical element to improvethe collimation of light emitted by an OLED array.

FIG. 9 is a diagram of a holographic display device.

FIG. 10 is a diagram of a holographic display device which incorporatestwo EASLMs for encoding amplitude and phase in succession.

FIG. 11 is a diagram of a holographic display device including a singleEASLM.

FIG. 12 is a diagram of a specific embodiment of a holographic displayaccording an implementation.

FIG. 13 is a diagram of a holographic display device which incorporatestwo EASLMs for encoding amplitude and phase in succession.

FIG. 14 is diffraction simulation results obtained using MathCad®.

FIG. 15 is diffraction simulation results obtained using MathCad®.

FIG. 16 is diffraction simulation results obtained using MathCad®.

FIG. 17 is an arrangement of two EASLMS with a lens layer between,according to an implementation.

FIG. 18 is a diagram of a diffraction process which may occur as lighttravels from one EASLM to a second EASLM.

FIG. 19 is a diagram of a configuration of two EASLMS, in which a fibreoptic face plate is situated between the two EASLMs.

FIG. 20 is a diagram of a beam steering element.

FIG. 21 is a diagram of a beam steering element.

FIG. 22 is a diagram of a system which enables visual communication inthree dimensions.

FIG. 23 is a diagram of a method for converting 2D image content to 3Dimage content.

FIG. 24 is a diagram of an embodiment of a holographic display componentaccording to an implementation.

FIG. 25 is a schematic drawing of a holographic display comprising lightsources in a 2D light source array, lenses in a 2D lens array, a SLM anda beamsplitter. The beamsplitter splits the rays leaving the SLM intotwo bundles each of which illuminates the virtual observer window forthe left eye (VOWL) and the virtual observer window for the right eye(VOWR), respectively.

FIG. 26 is a schematic drawing of a holographic display comprising twolight sources of a light source array, and two lenses of a lens array, aSLM and a beamsplitter. The beamsplitter splits the rays leaving the SLMinto two bundles each of which illuminates the virtual observer windowfor the left eye (VOWL) and the virtual observer window for the righteye (VOWR), respectively.

FIG. 27 is a cross-sectional diagram of a prismatic beam steeringelement.

DETAILED DESCRIPTION

Various implementations will now be described.

A. Compact Combination of an Infra Red OLED Display and OASLM

This implementation provides a compact combination of an OASLM and aninfra red light emitting display which can write a pattern on the OASLM,the combination being capable of generating a three dimensional imageunder suitable illumination conditions.

An OASLM comprises a photosensor layer and a liquid crystal (LC) layerwhich is located between conducting electrodes. When a voltage isapplied to the electrodes, a light pattern incident on the photosensorlayer is transferred to the LC layer where it is used to modulate a readbeam. In prior art, the incident light pattern is provided by a writebeam that is modulated by an electrically addressed spatial lightmodulator (EASLM). The EASLM is illuminated by a light source and imagedonto the OASLM. Usually, the write beam is incoherent to avoid specklepatterns whereas the read beam is coherent to enable the generation of adiffraction pattern.

An advantage of an OASLM with respect to an EASLM is that an OASLM mayhave a continuous, non-pixellated or non-patterned structure, whereas aEASLM has a pixellated structure. Pixels have sharp edges in the spatialdistribution of light they produce: such sharp edges correspond to highspatial frequencies. High spatial frequencies lead to broad anglediffraction features in the optical far field. Therefore an EASLM willproduce undesirable optical diffraction artefacts in the optical farfield, which have to be removed using known techniques such as spatialfiltering. Spatial filtering requires an additional step in the opticalprocessing procedure, which makes devices thicker and leads to wastedlight. An advantage of OASLM-based devices is that they permitcontinuous pattern production in the OASLM. A continuous pattern willtend to have less abrupt variations in the optical intensity in anygiven direction transverse to the beam propagation direction. The lessabrupt variations therefore possess a lower concentration of highspatial frequencies than in the case of pixel edges generated by anEASLM device. The reduced concentration of high spatial frequencies inthe case of an OASLM-containing device may make optical processingeasier and more efficient than in the case of an EASLM-containingdevice. In addition, an OASLM device may be a bistable device, incontrast to an EASLM. Therefore an OASLM may have lower powerrequirements than an EASLM device, which may increase the batterylifetime in a portable device, or in a handheld device.

In this implementation, a compact device with no requirement for imagingoptics is described. The OASLM is written with an infra red OLEDdisplay. The OLED display is directly attached to the OASLM, thusforming a compact device without imaging optics. OLEDs may be tiled soas to make up an OLED array. The OASLM may be made up from multiplesmaller tiled OASLMs.

The compact combination of the OLED display and the OASLM may betransparent. Transparent OLED-displays are known such as those describedin the “OLED MATERIALS” section below. In one example, the compactcombination of the OLED display and the OASLM is illuminated from theopposite side to the side on which the three dimensional image isformed, with visible light transmitted through the OLED and the OASLMtowards the observer. Preferably, the OLED display emits infrared light(IR) that writes the IR-sensitive photosensor layer of the OASLM.Because the human eye is not sensitive to IR light, the observer willnot see any light which originates from the IR write beam.

In another example, the compact combination of the OLED display and theOASLM may be such that the write beam and the read beam are incident onopposite sides of the OASLM. In another example, the compact combinationof the OLED display and the OASLM may be such that a reflective layer ispresent on the side of the OASLM which is the side opposite to the OLEDdisplay such that the three dimensional image is viewable from the sameside of the OASLM as the side on which the OLED display is present,where the illumination source is also present on the same side of theOASLM as the OLED display: this is an example of a reflective display.

An implementation includes an array of infra red OLEDs, the infra redemitting OLEDs permitting control of the spatial distribution of theamplitude or phase, or some combination of the amplitude and phase, ofthe visible light transmitted by an OASLM, such that a hologram isgenerated in the OASLM. An OASLM may comprise of a pair of spacedtransparent plates on which two electrically conducting films arecoated, as described in U.S. Pat. No. 4,941,735, which is incorporatedherein by reference. A continuous or discontinuous photosensitive filmmay be coated on one of the conductive films. A bistable ferroelectricliquid crystal, or some other type of liquid crystal, may be confinedbetween the other conductive film and the photosensitive film. Anactivating voltage may be applied to the conductive films. In an OASLM,an optical write beam may program or activate the polarization of anoptical read beam, on a pixel-by-pixel basis. The write beam may programthe OASLM by activating individually photosensitive areas of the OASLM.Areas of the OASLM that are programmed accordingly may rotate thepolarization of the read beam through being activated by the write beam.

In FIG. 1, an example of an implementation is disclosed. 10 is anillumination apparatus for providing illumination of a plane area, wherethe illumination has sufficient coherence so as to be able to lead tothe generation of a three dimensional image. An example of anillumination apparatus is disclosed in US 2006/250671 for the case oflarge area video holograms, one example of which is reproduced in FIG.4. Such an apparatus as 10 may take the form of an array of white lightsources, such as cold cathode fluorescent lamps or white-light lightemitting diodes which emit light which is incident on a focusing systemwhich may be compact, such as a lenticular array or a microlens array.Alternatively, light sources for 10 may comprise of red, green and bluelasers or red, green and blue light emitting diodes which emit light ofsufficient coherence. However, non-laser sources with sufficient spatialcoherence (eg. light emitting diodes, OLEDs, cold cathode fluorescentlamps) are preferred to laser sources. Laser sources have disadvantagessuch as causing laser speckle in the holographic reconstructions, beingrelatively expensive, and having possible safety problems with regard topossibly damaging the eyes of holographic display viewers or of thosewho work in assembling the holographic display devices.

Elements 10-13 may be about a few centimetres in thickness, or less, intotal. Element 11 may comprise of an array of colour filters, such thatpixels of coloured light, such as red, green and blue light, are emittedtowards element 12, although the colour filters may not be required ifcoloured sources of light are used. Element 12 is an array of infra redemitting OLEDs on a transparent substrate. The array of infra redemitting OLEDs is such that each infra red emitting OLED emits lightparallel to and coincident with the light from a unique correspondingcolour pixel, in the direction of element 13. Element 13 is an OASLM.With regard to the OASLM, the array of infra red emitting OLEDs suppliesthe write beams; the coloured beams transmitted by element 11 are theread beams. A viewer located at point 14 some distance from the devicewhich includes the compact hologram generator 15 may view a threedimensional image when viewing in the direction of 15. Elements 10, 11,12 and 13 are disposed so as to be in physical, e.g. actual mechanical,contact, each forming a layer of a structure so that the whole is asingle, unitary object. Physical contact may be direct. Or it may beindirect, if there is a thin, intervening layer, coating of film betweenadjacent layers. Physical contact may be limited to small regions thatensure correct mutual alignment or registration, or may extend to largerareas, or the entire surface of a layer. Physical contact may beachieved by layers being bonded together such as through the use of anoptically transmitting adhesive, so as to form a compact hologramgenerator 15, or by any other suitable process (see also section belowtitled Outline Manufacturing Process).

Element 10 may include one or two prismatic optical films for increasingdisplay brightness: such films are disclosed eg. in U.S. Pat. No.5,056,892 and in U.S. Pat. No. 5,919,551, though others are known.Element 10 may include a polarizing optical element, or a set ofpolarizing optical elements. One example is a linear polarizer sheet. Afurther example is a reflective polarizer which transmits one linearpolarization state and reflects the orthogonal linear polarizationstate—such a sheet is described in U.S. Pat. No. 5,828,488, for example,though others are known. A further example is a reflective polarizerwhich transmits one circular polarization state and reflects theorthogonal circular polarization state—such a sheet is described in U.S.Pat. No. 6,181,395, for example, though others are known. Element 10 mayinclude a focusing system which may be compact such as a lenticulararray or a microlens array. Element 10 may include other opticalelements which are known in the field of backlight technology.

FIG. 4 is a prior art side view showing three focusing elements 1101,1102, 1103 of a vertical focusing system 1104 in the form of cylindricallenses horizontally arranged in an array, taken from WO 2006/119920,which is incorporated herein by reference. The nearly collimated beamsof a horizontal line light source LS₂ passing through the focusingelement 1102 of an illumination unit and running to an observer plane OPare exemplified. According to FIG. 4, a multitude of line light sourcesLS₁, LS₂, LS₃ are arranged one above another. Each light source emitslight which is sufficiently spatially coherent in the vertical directionand which is spatially incoherent in the horizontal direction. Thislight passes through the transmissive cells of the light modulator SLM.The light is only diffracted in the vertical direction by cells of thelight modulator SLM, which are encoded with a hologram. The focusingelement 1102 images a light source LS₂ in the observer plane OP inseveral diffraction orders, of which only one is useful. The beamsemitted by the light source LS₂ are exemplified to pass only through thefocusing element 1102 of focusing system 1104. In FIG. 4 the three beamsshow the first diffraction order 1105, the zeroth order 1106 and theminus first order 1107. In contrast to a single point light source, aline light source allows the production of a significantly higherluminous intensity. Using several holographic regions with alreadyincreased efficiency and with the assignment of one line light sourcefor each portion of a 3D-scene to be reconstructed, improves theeffective luminous intensity. Another advantage is that, instead of alaser, a multitude of conventional light sources, which are positionede.g. behind a slot diaphragm, which may also be part of a shutter,generate sufficiently coherent light.

B. Compact Combination of Two Pairs of OLED and OASLM Combinations

In a further implementation, a combination of two pairs of a compactcombination of an OLED array and an OASLM can be used to modulate theamplitude and the phase of light in sequence and in a compact way. Thus,a complex number, which consists of an amplitude and a phase, can beencoded in the transmitted light, on a pixel by pixel basis.

This implementation comprises a first compact combination pair of anIR-OLED array and a OASLM and a second compact combination pair of anIR-OLED array and a OASLM.

The first pair modulates the amplitude of transmitted light and thesecond pair modulates the phase of the transmitted light. Alternatively,the first pair modulates the phase of transmitted light and the secondpair modulates the amplitude of the transmitted light. Each compactcombination pair of an IR-OLED array and a OASLM may be as described insection A above. The two compact combination pairs of an IR-OLED arrayand a OASLM are separated by an IR-filter that is transparent forvisible light and is absorbing for IR.

In a first step the first IR-OLED array writes the pattern for amplitudemodulation in the first OASLM. In a second step the second IR-OLED arraywrites the pattern for phase modulation in the second OASLM. TheIR-Filter prevents the leakage of IR from the first compact combinationpair of an IR-OLED array and an OASLM to the second compact combinationpair of an IR-OLED array and an OASLM. The IR-Filter also prevents theleakage of IR from the second compact combination pair of an IR-OLEDarray and an OASLM to the first compact combination pair of an IR-OLEDarray and an OASLM. However, the IR filter transmits the visible lightfrom the first compact combination pair of an IR-OLED array and an OASLMfor use as the read beam in the second compact combination pair of anIR-OLED array and an OASLM. The light transmitted by the second OASLMhas been modulated in its amplitude and in its phase as a result ofwhich an observer may observe a three dimensional image when viewing thelight emitted by the device in which the two compact combination pairsare housed.

It will be appreciated by those skilled in the art that the modulationof phase and amplitude facilitates the representation of complexnumbers. Furthermore, OLED-displays and OASLMs may both have highresolution. Therefore, this implementation may be used to generateholograms such that a three dimensional image may be viewed by a viewer.

In FIG. 2, an example of an implementation is disclosed. 20 is anillumination apparatus for providing illumination of a plane area, wherethe illumination has sufficient coherence so as to be able to lead tothe generation of a three dimensional image. An example is disclosed inUS 2006/250671 for the case of large area video holograms. Such anapparatus as 20 may take the form of an array of a white light sources,such as cold cathode fluorescent lamps or white light light emittingdiodes which emit light which is incident on a focusing system which maybe compact such as a lenticular array or a microlens array.Alternatively, light sources for 20 may comprise of red, green and bluelasers or red, green and blue light emitting diodes which emit light ofsufficient coherence. However, non-laser sources with sufficient spatialcoherence (eg. light emitting diodes, OLEDs, cold cathode fluorescentlamps) are preferred to laser sources. Laser sources have disadvantagessuch as causing laser speckle in the holographic reconstructions, beingrelatively expensive, and having possible safety problems with regard topossibly damaging the eyes of holographic display viewers or of thosewho work in assembling the holographic display devices.

Elements 20-23, 26-28 may be about a few centimetres in thickness, orless, in total. Element 21 may comprise of an array of colour filters,such that pixels of colour light, such as red, green and blue light, areemitted towards element 22, although the colour filters may not berequired if coloured sources of light are used. Element 22 is an arrayof infra red emitting OLEDs on a transparent substrate. The array ofinfra red emitting OLEDs is such that each infra red emitting OLED emitslight parallel to and coincident with the light from a uniquecorresponding colour pixel, in the direction of element 23. Element 23is an OASLM. With regard to the OASLM, the array of infra red emittingOLEDs supplies the write beams; the coloured beams transmitted byelement 21 are the read beams. Element 26 is an infra red filter whichblocks IR light but transmits visible light, such that IR light fromelement 22 does not influence element 27. Element 27 is an OASLM.Element 28 is an array of infra red emitting OLEDs on a transparentsubstrate. The array of infra red emitting OLEDs is such that each infrared emitting OLED emits light parallel to and coincident with the lightfrom a unique corresponding colour pixel, in the direction of element27. With regard to the OASLM 27, the array of infra red emitting OLEDs28 supplies the write beams; the coloured beams transmitted by element26 are the read beams. With regard to the transmitted light, element 23modulates the amplitude and element 27 modulates the phase.Alternatively, element 27 modulates the amplitude and element 23modulates the phase. Because the light from the array infra red emittingOLEDs on a transparent substrate 28 is emitted in the direction ofelement 26, element 26 may absorb IR light, which prevents light fromelement 28 from addressing OASLM 23. Such a configuration, in which thetwo OLED arrays 22 and 28 emit light in substantially oppositedirections, ensures that the two OASLMs 23 and 27 may be placed in closeproximity. The close proximity of OASLMs 23 and 27 enables a reductionin the problems of optical losses and pixel cross-talk arising fromoptical beam divergence: when OASLMs 23 and 27 are in closer proximity,a better approximation to non-overlapping propagation of the beams ofcoloured light through the OASLMs may be achieved. The order of elements27 and 28 may be reversed in FIG. 2, but this is not thought be theoptimal configuration for achieving the goals of high transmission ofand low cross-talk between the beams of coloured light through theOASLMs 23 and 27.

Element 20 may include one or two prismatic optical films for increasingdisplay brightness: such films are disclosed eg. in U.S. Pat. No.5,056,892 and in U.S. Pat. No. 5,919,551, though others are known.Element 20 may include a polarizing optical element, or a set ofpolarizing optical elements. One example is a linear polarizer sheet. Afurther example is a reflective polarizer which transmits one linearpolarization state and reflects the orthogonal linear polarizationstate—such a sheet is described in U.S. Pat. No. 5,828,488, for example,though others are known. A further example is a reflective polarizerwhich transmits one circular polarization state and reflects theorthogonal circular polarization state—such a sheet is described in U.S.Pat. No. 6,181,395, for example, though others are known. Element 20 mayinclude a focusing system which may be compact such as a lenticulararray or a microlens array. Element 20 may include other opticalelements which are known in the field of backlight technology.

A viewer located at point 24 some distance from the device whichincludes the compact hologram generator 25 may view a three dimensionalimage when viewing in the direction of 25. Elements 20, 21, 22, 23, 26,27 and 28 are arranged so that adjacent elements are in physical, e.g.fixed mechanical, contact, each forming a layer of a structure so thatthe whole is a single, unitary object. Physical contact may be direct.Or it may be indirect, if there is a thin, intervening layer, coating offilm between adjacent layers. Physical contact may be limited to smallregions that ensure correct mutual alignment or registration, or mayextend to larger areas, or the entire surface of a layer. Physicalcontact may be achieved by layers being bonded together such as throughthe use of an optically transmitting adhesive, so as to form a compacthologram generator 25, or by any other suitable process (see alsosection below titled Outline Manufacturing Process).

In FIG. 2, in the ideal case the arrays of OLEDs 22 and 28 emit lightthat is well-collimated. However, OLEDs may emit light that is not wellcollimated, such as light in a Lambertian (i.e. totally diffuse)distribution. Where the OLED light emission is not well-collimated, theOLEDs may be situated as close as possible to their corresponding OASLM.In this case, it is thought that the intensity incident on the OASLMsurface will vary approximately as the square of the cosine of the angleof incidence. Light incident at 45° or 60° will result in respectiveintensities of only a half or a quarter of normally incident light. Thusprovided that the OLEDs are sufficiently spaced apart, and sufficientlysmall with respect to the visible light pixel size, and sufficientlyclose to the OASLM, geometric effects will lead to significantvariations in the potential difference generated spatially across theOASLM, even in the limiting case that the OLED light emissiondistribution is Lambertian. The incident infra red light intensity maynot fall to zero in between the points on the OASLM where OLED light isnormally incident, which may lead to a reduction in the contrast whichcan be achieved in the device. But this contrast reduction may beacceptable if it simplifies device construction.

In FIG. 2, in the ideal case the arrays of OLEDs 22 and 28 emit lightthat is well-collimated. However, OLEDs may emit light that is not wellcollimated, such as light in a Lambertian (i.e. totally diffuse)distribution. Where the OLED light emission is not well-collimated, theOLEDs' geometric light distribution may be modified through the use of aBragg filter holographic optical element, such as described in U.S. Pat.No. 5,153,670 which is incorporated herein by reference. The Braggfilter holographic optical element results in light that is collimated,or better collimated, than in the absence of this element. An example ofthe functioning of the Bragg filter holographic optical element is shownin FIG. 8. In FIG. 8, 80 is an OLED array, 81 is a holographic opticalelement Bragg filter which contains Bragg planes such as Bragg plane 84,and 82 is an OASLM. A single OLED 83 in OLED array 80 emits infra redlight in a distribution indicated schematically as 85. A light ray 86 isindicated which is emitted by OLED array 80, undergoes scattering inholographic optical element 81, and is subsequently incident on OASLM 82at approximately normal incidence. In this manner, improved collimationof the infra red light that is incident on OASLM 82 may be achieved.

A further implementation is disclosed in FIG. 5. 57 is an illuminationapparatus for providing illumination of a plane area, where theillumination has sufficient coherence so as to be able to lead to thegeneration of a three dimensional image. An example is disclosed in US2006/250671 for the case of large area video holograms. Such anapparatus may take the form of an array of a white light sources, suchas cold cathode fluorescent lamps or white light light emitting diodeswhich emit light which is incident on a focusing system which may becompact such as a lenticular array or a microlens array 50.Alternatively, light sources for 57 may comprise of red, green and bluelasers or red, green and blue light emitting diodes which emit light ofsufficient coherence. However, non-laser sources with sufficient spatialcoherence (eg. light emitting diodes, OLEDs, cold cathode fluorescentlamps) are preferred to laser sources. Laser sources have disadvantagessuch as causing laser speckle in the holographic reconstructions, beingrelatively expensive, and having possible safety problems with regard topossibly damaging the eyes of holographic display viewers or of thosewho work in assembling the holographic display devices.

Element 57 may include one or two prismatic optical films for increasingdisplay brightness: such films are disclosed eg. in U.S. Pat. No.5,056,892 and in U.S. Pat. No. 5,919,551, though others are known.Element 57 may include a polarizing optical element, or a set ofpolarizing optical elements. One example is a linear polarizer sheet. Afurther example is a reflective polarizer which transmits one linearpolarization state and reflects the orthogonal linear polarizationstate—such a sheet is described in U.S. Pat. No. 5,828,488, for example,though others are known. A further example is a reflective polarizerwhich transmits one circular polarization state and reflects theorthogonal circular polarization state—such a sheet is described in U.S.Pat. No. 6,181,395, for example, though others are known. Element 57 mayinclude other optical elements which are known in the field of backlighttechnology.

Elements 57, 50-54 may be about a few centimetres in thickness, or less,in total. Element 51 may comprise of an array of colour filters, suchthat pixels of colour light, such as red, green and blue light, areemitted towards element 52, although the colour filters may not berequired if coloured sources of light are used. Element 52 is an arrayof infra red emitting OLEDs on a transparent substrate. The array ofinfra red emitting OLEDs is such that for each colour pixel, a uniquepair comprising of two types of infra red emitting OLED emit lightparallel to and coincident with the light from their correspondingcolour pixel, in the direction of element 53. The first type of infrared emitting OLED emits infra red light of a first wavelength. Thesecond type of infra red emitting OLED emits infra red light of a secondwavelength, which is different to the first wavelength. Element 53 is anOASLM. Element 54 is a further OASLM. With regard to the OASLMs, thearray of infra red emitting OLEDs supplies the write beams; the colouredbeams transmitted by element 51 are the read beams. OASLM 53 iscontrolled by the first wavelength of the two infra red wavelengthsemitted by the OLED array 52. OASLM 53 is insensitive to the secondwavelength of the two infra red wavelengths emitted by the OLED array52, and transmits the second wavelength of the two infra red wavelengthsemitted by the OLED array 52. OASLM 54 is controlled by the secondwavelength of the two infra red wavelengths emitted by the OLED array52. OASLM 54 is insensitive to the first wavelength of the two infra redwavelengths emitted by the OLED array 52, or light of the first infrared wavelength is prevented from reaching OASLM 54 through itsabsorption and/or reflection by OASLM 53, followed by its absorptionelsewhere, so that insensitivity of OASLM 54 to the first infra redwavelength is not necessarily a requirement of the compact hologramgenerator 55. Alternatively it may be possible to use a single type ofOLED which emits two different wavelengths, where the relative intensityof the two different wavelengths depends on a parameter such as thevoltage across the OLED. Emission of the two different wavelengths couldbe controlled by temporal multiplexing.

With regard to the transmitted light, element 53 modulates the amplitudeand element 54 modulates the phase. Alternatively, element 54 modulatesthe amplitude and element 53 modulates the phase. Such a configuration,in which the OLED array 52 emits light of two different wavelengths,ensures that the two OASLMs 53 and 54 may be placed in close proximity.The close proximity of OASLMs 53 and 54 enables a reduction in theproblems of optical losses and pixel cross-talk arising from opticalbeam divergence: when OASLMs 53 and 54 are in closer proximity, a betterapproximation to non-overlapping propagation of the beams of colouredlight through the OASLMs may be achieved.

A viewer located at point 56 some distance from the device whichincludes the compact hologram generator 55 may view a three dimensionalimage when viewing in the direction of 55. Elements 57, 50, 51, 52, 53,and 54 are arranged so that adjacent elements are in physical, e.g.fixed mechanical, contact, each forming a layer of a structure so thatthe whole is a single, unitary object. Physical contact may be direct.Or it may be indirect, if there is a thin, intervening layer, coating offilm between adjacent layers. Physical contact may be limited to smallregions that ensure correct mutual alignment or registration, or mayextend to larger areas, or the entire surface of a layer. Physicalcontact may be achieved by layers being bonded together such as throughthe use of an optically transmitting adhesive, so as to form a compacthologram generator 55, or by any other suitable process (see alsosection below titled Outline Manufacturing Process).

Where an OASLM performs amplitude modulation, in a typical configurationthe incident read optical beams will be linearly polarized by passingthe beams through a linear polarizer sheet. Amplitude modulation iscontrolled by the rotation of the liquid crystal in an applied electricfield, where the electric field is generated by the photosensitivelayer, which influences the polarization state of the light. In such adevice, the light which exits the OASLM is passed through a furtherlinear polarizer sheet, which enables intensity reduction as a result ofany change in the polarization state of the light as it passes throughthe OASLM.

Where an OASLM performs phase modulation, in a typical configuration theincident read optical beams will be linearly polarized by passing thebeams through a linear polarizer sheet, unless they are already in adefined linear polarization state. Phase modulation is controlled byapplication of an applied electric field, where the electric field isgenerated by the photosensitive layer, which influences the phase stateof the light. In one example of phase modulation implemented using anematic phase liquid crystal, the optic axis direction is fixed in spacebut the birefringence is a function of the applied voltage. In oneexample of phase modulation implemented using a ferroelectric liquidcrystal, the birefringence is fixed, but the direction of the optic axisis controlled by the applied voltage. In phase modulation implementedusing either method, the output beam has a phase difference with respectto the input beam that is a function of the applied voltage. An exampleof a liquid crystal cell which can perform phase modulation is aFreedericksz cell arrangement in which anti-parallel aligned domains ofa nematic liquid crystal with a positive dielectric anisotropy are used,as described in U.S. Pat. No. 5,973,817 which is incorporated herein byreference.

C. Compact Combination of an EASLM and a Compact Light Source

This implementation provides a compact combination of an EASLM and acompact light source of sufficient coherence, the combination beingcapable of generating a three dimensional image under suitableillumination conditions.

In this implementation, a compact combination of an EASLM and a compactlight source, with no requirement for imaging optics, is described. Thisimplementation provides a compact combination of a light source orsources, a focusing means, an electrically addressed spatial lightmodulator (EASLM) and an optional beam splitter element, the combinationbeing capable of generating a three dimensional image under suitableillumination conditions.

In FIG. 11, an example of an implementation is disclosed. 110 is anillumination apparatus for providing illumination of a plane area, wherethe illumination has sufficient coherence so as to be able to lead tothe generation of a three dimensional image. An example of anillumination apparatus is disclosed in US 2006/250671 for the case oflarge area video holograms, one example of which is reproduced in FIG.4. Such an apparatus as 110 may take the form of an array of white lightsources, such as cold cathode fluorescent lamps or white light lightemitting diodes which emit light which is incident on a focusing systemwhich may be compact, such as a lenticular array or a microlens array.Alternatively, light sources for 110 may comprise of red, green and bluelasers or red, green and blue light emitting diodes which emit light ofsufficient coherence. The red, green and blue light emitting diodes maybe organic light emitting diodes (OLEDs). However, non-laser sourceswith sufficient spatial coherence (eg. light emitting diodes, OLEDs,cold cathode fluorescent lamps) are preferred to laser sources. Lasersources have disadvantages such as causing laser speckle in theholographic reconstructions, being relatively expensive, and havingpossible safety problems with regard to possibly damaging the eyes ofholographic display viewers or of those who work in assembling theholographic display devices.

Element 110 may be about a few centimetres in thickness, or less. In apreferred embodiment, elements 110-113 in total are less than 3 cm inthickness, so as to provide a compact source of light of sufficientcoherence. Element 111 may comprise of an array of colour filters, suchthat pixels of coloured light, such as red, green and blue light, areemitted towards element 112, although the colour filters may not berequired if coloured sources of light are used. Element 112 is an EASLM.Element 113 is an optional beamsplitter element. A viewer located atpoint 114 some distance from the device which includes the compacthologram generator 115 may view a three dimensional image when viewingin the direction of 115.

Element 110 may include one or two prismatic optical films forincreasing display brightness: such films are disclosed eg. in U.S. Pat.No. 5,056,892 and in U.S. Pat. No. 5,919,551, though others are known.Element 110 may include a polarizing optical element, or a set ofpolarizing optical elements. One example is a linear polarizer sheet. Afurther example is a reflective polarizer which transmits one linearpolarization state and reflects the orthogonal linear polarizationstate—such a sheet is described in U.S. Pat. No. 5,828,488, for example,though others are known. A further example is a reflective polarizerwhich transmits one circular polarization state and reflects theorthogonal circular polarization state—such a sheet is described in U.S.Pat. No. 6,181,395, for example, though others are known. Element 110may include other optical elements which are known in the field ofbacklight technology.

An EASLM is a SLM in which each cell in an array of cells may beaddressed electrically. Each cell acts on the light incident on it someway, such as to modulate the amplitude of the light it transmits, or tomodulate the phase of the light it transmits, or to modulate somecombination of the amplitude and phase of the light it transmits. Anexample of an EASLM is given in U.S. Pat. No. 5,973,817, which isincorporated herein by reference, the example being a phase modulatingEASLM. A liquid crystal EASLM is an example of a EASLM. A magnetooptical EASLM is a further example of an EASLM.

Elements 110, 111, 112 and 113 are disposed so as to be in physical,e.g. actual mechanical, contact, each forming a layer of a structure sothat the whole is a single, unitary object. Physical contact may bedirect. Or it may be indirect, if there is a thin, intervening layer,coating of film between adjacent layers. Physical contact may be limitedto small regions that ensure correct mutual alignment or registration,or may extend to larger areas, or the entire surface of a layer.Physical contact may be achieved by layers being bonded together such asthrough the use of an optically transmitting adhesive, so as to form acompact hologram generator 115, or by any other suitable process (seealso section below titled Outline Manufacturing Process).

FIG. 4 is a prior art side view showing three focusing elements 1101,1102, 1103 of a vertical focusing system 1104 in the form of cylindricallenses horizontally arranged in an array. The nearly collimated beams ofa horizontal line light source LS₂ passing through the focusing element1102 of an illumination unit and running to an observer plane OP areexemplified. According to FIG. 4, a multitude of line light sources LS₁,LS₂, LS₃ are arranged one above another. Each light source emits lightwhich is sufficiently coherent in the vertical direction and which isincoherent in the horizontal direction. This light passes through thetransmissive cells of the light modulator SLM. The light is onlydiffracted in the vertical direction by cells of the light modulatorSLM, which are encoded with a hologram. The focusing element 1102 imagesa light source LS₂ in the observer plane OP in several diffractionorders, of which only one is useful. The beams emitted by the lightsource LS₂ are exemplified to pass only through the focusing element1102 of focusing system 1104. In FIG. 4 the three beams show the firstdiffraction order 1105, the zeroth order 1106 and the minus first order1107. In contrast to a single point light source, a line light sourceallows the production of a significantly higher luminous intensity.Using several holographic regions with already increased efficiency andwith the assignment of one line light source for each portion of a3D-scene to be reconstructed, improves the effective luminous intensity.Another advantage is that, instead of a laser, a multitude ofconventional light sources, which are positioned e.g. behind a slotdiaphragm, which may also be part of a shutter, generate sufficientlycoherent light.

In general, a holographic display is used to reconstruct a wavefront ina virtual observer window. The wavefront is one that a real object wouldgenerate, if it were present. An observer sees the reconstructed objectwhen his eyes are positioned at an virtual observer window, which may beone of several possible virtual observer windows (VOWs). As shown inFIG. 6A, the holographic display comprises the following components: alight source, a lens, a SLM, and an optional beam splitter.

In order to facilitate the creation of a compact combination of a SLMand a compact light source which may display holographic images, thesingle light source and the single lens of FIG. 6A may be replaced by alight source array and a lens array or a lenticular array, respectively,as shown in FIG. 6B. In FIG. 6B, the light sources illuminate the SLMand the lenses image the light sources into the observer plane. The SLMis encoded with a hologram and modulates the incoming wavefront suchthat the desired wavefront may be reconstructed in the VOW. An optionalbeam splitter element may be used to generate several VOWs, e.g. one VOWfor the left eye and one VOW for the right eye.

If a light source array and a lens array or a lenticular array are used,the light sources in the array have to be positioned such that the lightbundles through all the lenses of the lens array or lenticular arraycoincide in the VOW.

The apparatus of FIG. 6B lends itself to a compact design that can beused for a compact holographic display. Such a holographic display maybe useful for mobile applications, e.g. in a mobile phone or a PDA.Typically, such a holographic display would have a screen diagonal ofthe order of one inch or several inches. A holographic sub-display couldhave a screen diagonal as small as one cm. The appropriate componentsare described in detail below.

1) Light Source/Light Source Array

In a simple case, a fixed single light source can be used. If anobserver moves, the observer may be tracked, and the display may beadjusted so as to create an image which is viewable at the new positionof the observer. Here, there is either no tracking of the VOW ortracking is performed using a beam steering element after the SLM.

A configurable light source array may be achieved by a liquid crystaldisplay (LCD) that is illuminated by a backlight. Only the appropriatepixels are switched to the transmission state in order to create anarray of point or line light sources. The apertures of these lightsources have to be sufficiently small to guarantee sufficient spatialcoherence for holographic reconstruction of an object. An array of pointlight sources may be used in combination with a lens array thatcomprises a 2D arrangement of lenses. An array of line light sources ispreferably used in combination with a lenticular array that comprises aparallel arrangement of cylindrical lenses.

Preferably, an OLED display is used as a light source array. As aself-emitting device, it is more compact and more energy-efficient thana LCD where most of the light generated is absorbed by elements such ascolour filters or in pixels that are not in a fully transmissive state.However, LCDs may have an overall cost advantage over OLED displays,even when one allows for the situation in which OLED displays providelight in a more energy efficient way than LCD displays. When an OLEDdisplay is used as a light source array only those pixels are switchedon that are necessary for generating the VOW at the eye positions. TheOLED display may have a 2D arrangement of pixels or a 1D arrangement ofline light sources. The emitting area of each point light source or thewidth of each line light source has to be sufficiently small toguarantee sufficient spatial coherence for holographic reconstruction ofan object. Again, an array of point light sources is preferably used incombination with a lens array that comprises a 2D arrangement of lenses.An array of line light sources is preferably used in combination with alenticular array that comprises a parallel arrangement of cylindricallenses.

2) Focusing Means: Single Lens, Lens Array or Lenticular Array

The focusing means images the light source or the light sources to theobserver plane. As the SLM is in close proximity to the focusing means,the Fourier transform of the information encoded in the SLM is in theobserver plane. The focusing means comprises one or several focusingelements. The positions of SLM and of the focusing means may be swapped.

For a compact combination of an EASLM and a compact light source ofsufficient coherence, it is essential to have a thin focusing means: aconventional refractive lens with a convex surface would be too thick.Instead, a diffractive or a holographic lens may be used. Thisdiffractive or holographic lens may have the function of a single lens,of a lens array or of a lenticular array. Such materials are availableas surface relief holographic products supplied by Physical OpticsCorporation, Torrance, Calif., USA. Alternatively, a lens array may beused. A lens array comprises a 2D arrangement of lenses, where each lensis assigned to one light source of the light source array. In anotheralternative, a lenticular array may be used. A lenticular arraycomprises a 1D arrangement of cylindrical lenses, where each lens has acorresponding light source in the light source array. As mentionedabove, if a light source array and a lens array or a lenticular arrayare used, the light sources in the array have to be positioned such thatthe light bundles through all the lenses of the lens array or thelenticular array coincide in the VOW.

The light through the lenses of the lens array or the lenticular arrayis incoherent for one lens with respect to any other lens. Therefore thehologram that is encoded on the SLM is composed of sub-holograms, whereeach sub-hologram corresponds to one lens. The aperture of each lens hasto be sufficiently large to guarantee sufficient resolution of thereconstructed object. One may use lenses with an aperture that isapproximately as large as the typical size of an encoded area in thehologram, as has been described in US2006/0055994 for example. Thismeans that each lens should have an aperture of the order of one orseveral millimeters.

3) SLM

The hologram is encoded on the SLM. Usually, the encoding for a hologramconsists of a 2D array of complex numbers. Hence, ideally the SLM wouldbe able to modulate the amplitude and the phase of the local light beamspassing through each pixel of the SLM. However, a typical SLM is capableof modulating either amplitude or phase and not amplitude and phaseindependently.

An amplitude-modulating SLM may be used in combination with detour-phaseencoding, e.g. Burckhardt encoding. Its drawbacks are that three pixelsare needed to encode one complex number and the reconstructed object hasa low brightness.

A phase-modulating SLM results in a reconstruction with higherbrightness. As an example, a so-called 2-phase encoding may be used thatneeds two pixels to encode one complex number.

Although EASLMs have the property of sharply-defined edges, which leadto unwanted higher diffraction orders in their diffraction patterns, theuse of soft apertures can reduce or eliminate this problem. Softapertures are apertures without a sharp transmission cut off. An exampleof a soft aperture transmission function is one with a Gaussian profile.Gaussian profiles are known to be advantageous in diffractive systems.The reason is that there is a mathematical result that the Fouriertransform of a Gaussian function is itself a Gaussian function. Hencethe beam intensity profile function is unchanged by diffraction, exceptfor a lateral scaling parameter, in contrast to the case fortransmission through an aperture with a sharp cut-off in itstransmission profile. Sheet arrays of Gaussian transmission profiles maybe provided. When these are provided in alignment with the EASLMapertures, a system is provided in which higher diffraction orders willbe absent, or will be significantly reduced, compared with systems witha sharp cut off in the beam transmission profiles. The Gaussian filteror soft aperture filter suppresses diffraction artefacts from highspatial frequencies. The Gaussian filter or soft aperture filterminimizes crosstalk between virtual observer windows for the left andright eyes.

4) Beam Splitter Element

The VOW is limited to one periodicity interval of the Fourier transformof the information encoded in the SLM. With the currently available SLMsof maximum resolution, the size of the VOW is of the order of 10 mm. Insome circumstances, this may be too small for application in aholographic display without tracking. One solution to this problem isspatial multiplexing of VOWs: more than one VOWs are generated. In thecase of spatial multiplexing the VOWs are generated simultaneously fromdifferent locations on the SLM. This may be achieved by beam splitters.As an example, one group of pixels on the SLM is encoded with theinformation of VOW1, another group with the information of VOW2. A beamsplitter separates the light from these two groups such that VOW1 andVOW2 are juxtaposed in the observer plane. A larger VOW may be generatedby seamless tiling of VOW1 and VOW2. Multiplexing may also be used forgeneration of VOWs for the left and the right eye. In that case,seamless juxtaposition is not required and there may be a gap betweenone or several VOWs for the left eye and one or several VOWs for theright eye. Care has to be taken that higher diffraction orders of oneVOW do not overlap in the other VOWs.

A simple example of a beam splitter element is a parallax barrierconsisting of black stripes with transparent regions in between, asdescribed in US2004/223049 which is incorporated herein by reference. Afurther example is a lenticular sheet, as described in US2004/223049.Further examples of beam splitter elements are lens arrays and prismmasks. In a compact holographic display, one would typically expect abeam splitter element to be present, as the typical virtual observerwindow size of 10 mm would only be large enough for one eye, which isunsatisfactory as the typical viewer has two eyes which areapproximately 10 cm apart. However, as an alternative to spatialmultiplexing, temporal multiplexing may be used. In the absence ofspatial multiplexing, a beam splitter element does not have to be used.

Spatial multiplexing may also be used for the generation of colorholographic reconstructions. For spatial color multiplexing there areseparate groups of pixels for each of the color components red, greenand blue. These groups are spatially separated on the SLM and aresimultaneously illuminated with red, green and blue light. Each group isencoded with a hologram calculated for the respective color component ofthe object. Each group reconstructs its color component of theholographic object reconstruction.

5) Temporal Multiplexing

In the case of temporal multiplexing the VOWs are generated sequentiallyfrom the same location on the SLM. This may be achieved by alternatingpositions of the light sources and synchronously re-encoding the SLM.The alternating positions of the light sources have to be such thatthere is seamless juxtaposition of the VOWs in the observer plane. Ifthe temporal multiplexing is sufficiently fast, i.e. >25 Hz for thecomplete cycle, the eye will see a continuous enlarged VOW.

Multiplexing may also be used for generation of VOWs for the left andthe right eye. In that case, seamless juxtaposition is not required andthere may be a gap between one or several VOWs for the left eye and oneor several VOWs for the right eye. This multiplexing may be spatial ortemporal.

Spatial and temporal multiplexing may also be combined. As an example,three VOWs are spatially multiplexed to generate an enlarged VOW for oneeye. This enlarged VOW is temporally multiplexed to generate an enlargedVOW for the left eye and an enlarged VOW for the right eye.

Care has to be taken that higher diffraction orders of one VOW do notoverlap in the other VOWs.

Multiplexing for the enlargement of VOWs is preferably used withre-encoding of the SLM as it provides an enlarged VOW with continuousvariation of parallax upon observer motion. As a simplification,multiplexing without re-encoding would provide repeated content indifferent parts of the enlarged VOW.

Temporal multiplexing may also be used for the generation of colorholographic reconstructions. For temporal multiplexing the holograms forthe three color components are sequentially encoded on the SLM. Thethree light sources are switched synchronously with the re-encoding onthe SLM. The eye sees a continuous color reconstruction if the completecycle is repeated sufficiently fast, i.e. with >25 Hz.

6) Dealing with Unwanted Higher Diffraction Orders

If a larger VOW is generated by the tiling of smaller VOWs, higherdiffraction orders of one VOW may lead to a disturbing crosstalk inother VOWs unless steps are taken to avoid this problem. As an example,if each VOW is located in the zeroth diffraction order of the Fouriertransform of the information encoded in the SLM, the first diffractionorder of one VOW may overlap with an adjacent VOW. This overlap may leadto a disturbing background, which may become especially apparent if theintensity of the unwanted image exceeds about 5% of the intensity of thedesired image. In that case it is desirable to compensate for or tosuppress higher diffraction orders.

A static angular filter can be used if the angle with which the SLM isilluminated remains constant. This is the case if either the holographicdisplay has no tracking or the beam splitter element, such as a beamsteering element, is located after the SLM. The static angular filtermay be a Bragg filter or a Fabry Perot Etalon.

Where the SLM results in a geometric light intensity distributioncontaining unwanted diffraction orders, the geometric light intensitydistribution may be modified through the use of a Bragg filterholographic optical element, such as described in U.S. Pat. No.5,153,670. The Bragg filter holographic optical element results in alight intensity distribution that is different to the light intensitydistribution in the absence of this element. An example of thefunctioning of the Bragg filter holographic optical element is shown inFIG. 7. In FIG. 7, 70 is an SLM, 71 is a holographic optical elementBragg filter which contains Bragg planes such as Bragg plane 74. Asingle cell 73 in SLM 70 contributes to a diffracted light intensitydistribution indicated schematically as 75. A light ray 76 is indicatedwhich is diffracted by SLM 70, undergoes scattering in holographicoptical element 71, and is subsequently transmitted in a differentdirection to its original propagation direction between 70 and 71. Ifthe direction of light ray 76 propagation between 70 and 71 correspondsto unwanted first order diffracted light, it is clear that Bragg filter71 has succeeded in redirecting this light to a different direction,where it may not contribute to unwanted optical artefacts which mightdisturb a viewer, who typically will be located in the directionapproximately normal to 70.

A tunable Fabry Perot Etalon for the suppression of diffraction ordersis disclosed in patent application number DE 10 2006 030 503. What isdisclosed is a LC layer between two coplanar glass sheets that arecoated with a partially reflective coating. At each reflection of alight beam at the coatings the beam is partially reflected and partiallytransmitted. The transmitted beams interfere and the phase differencebetween them determines whether the interference is constructive ordestructive, as in a standard Fabry Perot Etalon. For a given wavelengththe interference and hence the transmission varies with the incidenceangle of the beam. For a given light propagation direction, theinterference can be tuned by varying the refractive index of the LC forthe given light propagation direction. The refractive index iscontrolled by an electric field applied across the LC layer. Therefore,the angular transmission characteristics can be tuned and diffractionorders can be selected for transmission, or for reflection, as required,within the overall constraints of the Fabry Perot Etalon. For example,if the Fabry Perot Etalon is configured for optimum transmission of thezeroth order, and optimum reflection of the first order, there may stillbe some unwanted transmission of the second order and higher orders.This device facilitates static or sequential selection of specificdiffraction orders that are transmitted, or reflected, as required,within the overall constraints of the Fabry Perot Etalon.

Spatial filters may be used to select diffraction orders. These spatialfilters may be located between the SLM and the VOW and comprisetransparent and opaque areas. These spatial filters may be used totransmit desired diffraction orders while blocking unwanted diffractionorders. These spatial filters may be static or configurable. Forexample, an EASLM placed between the SLM and the VOW may act as aconfigurable spatial filter.

7) Eye Tracking

In a compact combination of an EASLM and a compact light source ofsufficient coherence with eye tracking, an eye position detector maydetect the positions of the observer's eyes. One or several VOWs arethen automatically positioned at the eye positions so that the observercan see the reconstructed object through the VOWs.

However, tracking may not always be practical, especially for portabledevices, or in handheld devices, because of the constraints of theadditional apparatus required and electrical power requirements for itsperformance. Without tracking, the observer has to manually adjust theposition of the display. This is readily performed as in a preferredembodiment the compact display is a hand-held display that may beincorporated in a PDA or a mobile phone. As the user of a PDA or mobilephone usually tends to look perpendicularly on the display there is notmuch additional effort to align the VOWs with the eyes. It is known thata user of a hand-held device will tend automatically to orient thedevice in the hand so as to achieve the optimum viewing conditions, asdescribed for example in WO01/96941 which is incorporated herein byreference. Therefore, in such devices there is no necessity for user eyetracking and for complicated and non-compact tracking optics comprisingscanning mirrors, for example. But eye tracking could be implemented forsuch devices if the additional requirements for apparatus and electricalpower do not impose an excessive burden.

Without tracking, a compact combination of an EASLM and a compact lightsource of sufficient coherence requires VOWs that are sufficiently largein order to simplify the adjusting of the display. Preferably the VOWsize should be several times the size of the eye pupil. This can beachieved by either a single large VOW, using a SLM with a small pitch,or by the tiling of several small VOWs, using a SLM with a large pitch.

The position of the VOWs is determined by the positions of the lightsources in the light source array. An eye position detector detects thepositions of the eyes and sets the positions of the light sources inorder to adapt the VOWs to the eye positions. This kind of tracking isdescribed in US2006/055994 and in US2006/250671.

Alternatively, VOWs may be moved when the light sources are in fixedpositions. Light source tracking requires a SLM that is relativelyinsensitive to the variation of the incidence angle of light from thelight sources. If the light source is moved in order to move the VOWposition, this may be difficult to achieve with a compact combination ofa compact light source and a SLM due to the possible off-normal lightpropagation conditions within the compact combination that such aconfiguration implies. In such a case it is advantageous to have aconstant optical path in the display and a beam steering element as thelast optical component in the display.

A beam steering element which can provide these properties is shown inFIGS. 20 and 21. This beam steering element varies the angle of thelight bundles at the output of the display. It may have the opticalproperties of a controllable prism for x- and y-tracking and/or of acontrollable lens for z-tracking. For example, either of the beamsteering elements of FIGS. 20 and 21, or both, may be used within asingle device. The beam steering element is a controllable diffractiveelement or a controllable refractive element. A controllable refractiveelement may comprise an array of cavities filled with liquid crystalsthat are embedded in a matrix with an isotropic linear electric dipolesusceptibility tensor. The cavity has the shape of a prism or a lens.Application of an electric field controls the effective refractive indexof the liquid crystals and hence facilitates beam steering. The electricfield may be varied across the element to create beam steeringproperties which vary across the element. The electric field is appliedbetween the transparent electrodes shown in FIG. 20. The liquid crystalhas uniaxial refractive properties, and may be selected so that therefractive index perpendicular to its optic axis is equal to therefractive index of the host material, or “matrix”. Other configurationswill be obvious to those skilled in the art. The host material has anisotropic refractive index. If the optic axis of the liquid crystal isaligned with the z direction shown in FIG. 20 by application of asuitable electric field, a plane wave propagating along the z directionwill experience no refraction as it passes through the beam steeringelement, because it does not experience any refractive index variationperpendicular to its Poynting vector. However, if an electric field isapplied across the electrodes such that the optic axis of the liquidcrystals is perpendicular to the z direction, a plane wave propagatingalong the z direction which is polarized parallel to the optic axis willexperience maximum refraction as it passes through the beam steeringelement, because it experiences the maximum possible refractive indexvariation along its direction of polarization that the system canprovide. The degree of refraction will be tunable between these twoextreme cases through selection of an appropriate electric field acrossthe host material.

Beam steering may be accomplished if the cavities are prism-shaped,rather than lens-shaped. A suitable prism shape for beam steering isshown in FIG. 21. If the optic axis of the liquid crystal is alignedwith the z direction shown in FIG. 21 through the application of asuitable electric field, a plane wave propagating along the z directionwill experience no refraction as it passes through the beam steeringelement, because it does not experience any refractive index variationin its direction of polarization. However, if an electric field isapplied across the electrodes such that the optic axis of the liquidcrystals is perpendicular to the z direction, a plane wave propagatingalong the z direction which is polarized parallel to the optic axis willexperience maximum refraction as it passes through the beam steeringelement, because it experiences the maximum possible refractive indexvariation perpendicular to its Poynting vector that the system canprovide. The degree of refraction will be tunable between these twoextreme cases through selection of an appropriate electric field acrossthe host material.

8) Example

An example will now be described of a compact combination of an EASLMand a compact light source of sufficient coherence, the combinationbeing capable of generating a three dimensional image under suitableillumination conditions, that may be incorporated in a PDA or a mobilephone. The compact combination of an EASLM and a compact light source ofsufficient coherence comprises an OLED display as the light sourcearray, an EASLM and a lens array, as shown in FIG. 12.

Depending on the required position of the VOW (denoted OW in FIG. 12),specific pixels in the OLED display are activated. These pixelsilluminate the EASLM and are imaged into the observer plane by the lensarray. At least one pixel per lens of the lens array is activated in theOLED display. With the dimensions given in the drawing, the VOW can betracked with a lateral increment of 400 μm if the pixel pitch is 20 μm.This tracking is quasi-continuous.

An OLED pixel is a light source with only partial spatial coherence.Partial coherence leads to a smeared reconstruction of the objectpoints. With the dimensions given in the drawing, an object point at adistance of 100 mm from the display is reconstructed with a lateralsmearing of 100 μm if the pixel width is 20 μm. This is sufficient forthe resolution of the human vision system.

There is no significant mutual coherence between the light that passesthrough different lenses of the lens array. The coherence requirement islimited to each single lens of the lens array. Therefore, the resolutionof a reconstructed object point is determined by the pitch of the lensarray. A typical lens pitch will therefore be of the order of 1 mm toguarantee sufficient resolution for the human vision system. If the OLEDpitch is 20 μm, this means that the ratio of the lens pitch to the OLEDpitch is 50:1. If only a single OLED is lit per lens, this means thatonly one OLED in every 50^2=2,500 OLEDs will be lit. Hence the displaywill be a low power display. A difference between the holographicdisplays herein and a conventional OLED display is that the formerconcentrate the light at the viewer's eyes, whereas the latter emitslight into 2π steradians. Whereas a conventional OLED display achieves aluminance of about 1,000 cd/m^2, the inventors calculate that in thisimplementation, the illuminated OLED should achieve a luminance ofseveral times 1,000 cd/m^2 for practical application.

The VOW is limited to one diffraction order of the Fourier spectrum ofthe information encoded in the SLM. At a wavelength of 500 nm the VOWhas a width of 10 mm if the pixel pitch of the SLM is 10 μm and twopixels are needed to encode one complex number i.e. if 2-phase encodingon a phase-modulating EASLM is used. The VOW may be enlarged by tilingof VOWs by spatial or temporal multiplexing. In the case of spatialmultiplexing additional optical elements such as beam splitters arerequired.

Color holographic reconstructions can be achieved by temporalmultiplexing. The red, green and blue pixels of a color OLED display aresequentially activated with synchronous re-encoding of the SLM withholograms calculated for red, green and blue optical wavelengths.

The display may comprise an eye position detector that detects thepositions of the observer's eyes. The eye position detector is connectedwith a control unit that controls the activation of pixels of the OLEDdisplay.

The calculation of the holograms that are encoded on the SLM ispreferably performed in an external encoding unit as it requires highcomputational power. The display data are then sent to the PDA or mobilephone to enable the display of a holographically-generated threedimensional image.

As a practical example, a 2.6 inch screen diagonal XGA LCD EASLM made bySanyo® Epson® Imaging Devices Corporation of Japan may be used. Thesubpixel pitch is 17 μm. If this is used in constructing an RGBholographic display, with amplitude modulation encoding of the hologram,at a distance of 0.4 m from the EASLM the viewing window is calculatedto be 1.3 mm across. For the monochrome case, the viewing window iscalculated to be 4 mm across. If the same configuration is used, but itis implemented using phase modulation with two-phase encoding, theviewing window is calculated to be 6 mm across. If the sameconfiguration is used, but it is implemented using phase modulation withKinoform encoding, the viewing window is calculated to be 12 mm across.

Other high resolution EASLM examples exist. Seiko® Epson® Corporation ofJapan has released monochrome EASLMs, such as the D4:L3D13U1.3 inchscreen diagonal panel with a pixel pitch of 15 μm. The same company hasreleased a D5: L3D09U-61G00 panel in the same panel family with a screendiagonal length of 0.9 inches and a pixel pitch of 10 μm. On Dec. 12,2006 the same company announced the release of a L3D07U-81G00 panel inthe same family with a screen diagonal length of 0.7 inches and a pixelpitch of 8.5 μm. If the D4:L3D13U1.3 inch panel is used in constructinga monochrome holographic display, with Burckhardt amplitude modulationencoding of the hologram, at a distance of 0.4 m from the EASLM the VOWis calculated to be 5.6 mm across.

D. Compact Combination of a Pair of EASLMs

In a further implementation, a combination of two EASLMs can be used tomodulate the amplitude and the phase of light in sequence and in acompact way. Thus, a complex number, which consists of an amplitude anda phase, can be encoded in the transmitted light, on a pixel by pixelbasis.

This implementation comprises a compact combination of two EASLMs. Thefirst EASLM modulates the amplitude of transmitted light and the secondEASLM modulates the phase of the transmitted light. Alternatively, thefirst EASLM modulates the phase of transmitted light and the secondEASLM modulates the amplitude of the transmitted light. Each EASLM maybe as described in section C above. An overall assembly may be asdescribed in the section C, except two EASLMs are used here. Any othercombination of modulation characteristics of the two EASLMs is possiblethat is equivalent to facilitating independent modulation of amplitudeand phase.

In a first step the first EASLM is encoded with the pattern foramplitude modulation. In a second step the second EASLM is encoded withthe pattern for phase modulation. The light transmitted by the secondEASLM has been modulated in its amplitude and in its phase as a resultof which an observer may observe a three dimensional image when viewingthe light emitted by the device in which the two EASLMs are housed.

It will be appreciated by those skilled in the art that the modulationof phase and amplitude facilitates the representation of complexnumbers. Furthermore, EASLMs may have high resolution. Therefore, thisimplementation may be used to generate holograms such that a threedimensional image may be viewed by a viewer.

In FIG. 13, an example of an implementation is disclosed. 130 is anillumination apparatus for providing illumination of a plane area, wherethe illumination has sufficient coherence so as to be able to lead tothe generation of a three dimensional image. An example of anillumination apparatus is disclosed in US 2006/250671 for the case oflarge area video holograms, one example of which is reproduced in FIG.4. Such an apparatus as 130 may take the form of an array of white lightsources, such as cold cathode fluorescent lamps or white light lightemitting diodes which emit light which is incident on a focusing systemwhich may be compact, such as a lenticular array or a microlens array.Alternatively, light sources for 130 may comprise of red, green and bluelasers or red, green and blue light emitting diodes which emit light ofsufficient coherence. The red, green and blue light emitting diodes maybe organic light emitting diodes (OLEDs). However, non-laser sourceswith sufficient spatial coherence (eg. light emitting diodes, OLEDs,cold cathode fluorescent lamps) are preferred to laser sources. Lasersources have disadvantages such as causing laser speckle in theholographic reconstructions, being relatively expensive, and havingpossible safety problems with regard to possibly damaging the eyes ofholographic display viewers or of those who work in assembling theholographic display devices.

Element 130 may include one or two prismatic optical films forincreasing display brightness: such films are disclosed eg. in U.S. Pat.No. 5,056,892 and in U.S. Pat. No. 5,919,551, though others are known.Element 130 may include a polarizing optical element, or a set ofpolarizing optical elements. One example is a linear polarizer sheet. Afurther example is a reflective polarizer which transmits one linearpolarization state and reflects the orthogonal linear polarizationstate—such a sheet is described in U.S. Pat. No. 5,828,488, for example,though others are known. A further example is a reflective polarizerwhich transmits one circular polarization state and reflects theorthogonal circular polarization state—such a sheet is described in U.S.Pat. No. 6,181,395, for example, though others are known. Element 130may include a focusing system which may be compact such as a lenticulararray or a microlens array. Element 130 may include other opticalelements which are known in the field of backlight technology.

Element 130 may be about a few centimetres in thickness, or less. In apreferred embodiment, elements 130-134 are less than 3 cm in thicknessin total, so as to provide a compact source of light of sufficientcoherence. Element 131 may comprise of an array of colour filters, suchthat pixels of coloured light, such as red, green and blue light, areemitted towards element 132, although the colour filters may not berequired if coloured sources of light are used. Element 132 is an EASLM.Element 133 is an EASLM. Element 134 is an optional beamsplitterelement. With regard to the transmitted light, element 132 modulates theamplitude and element 133 modulates the phase. Alternatively, element133 modulates the amplitude and element 132 modulates the phase. Theclose proximity of EASLMs 132 and 133 enables a reduction in theproblems of optical losses and pixel cross-talk arising from opticalbeam divergence: when EASLMs 132 and 133 are in closer proximity, abetter approximation to non-overlapping propagation of the beams ofcoloured light through the EASLMs may be achieved. A viewer located atpoint 135 some distance from the device which includes the compacthologram generator 136 may view a three dimensional image when viewingin the direction of 136.

Elements 130, 131, 132, 133 and 134 are arranged so that adjacentelements are in physical, e.g. fixed mechanical, contact, each forming alayer of a structure so that the whole is a single, unitary object.Physical contact may be direct. Or it may be indirect, if there is athin, intervening layer, coating of film between adjacent layers.Physical contact may be limited to small regions that ensure correctmutual alignment or registration, or may extend to larger areas, or theentire surface of a layer. Physical contact may be achieved by layersbeing bonded together such as through the use of an opticallytransmitting adhesive, so as to form a compact hologram generator 136,or by any other suitable process (see also section below titled OutlineManufacturing Process).

Where an EASLM performs amplitude modulation, in a typical configurationthe incident read optical beams will be linearly polarized by passingthe beams through a linear polarizer sheet. Amplitude modulation iscontrolled by the rotation of the liquid crystal in an applied electricfield, which influences the polarization state of the light. In such adevice, the light which exits the EASLM is passed through a furtherlinear polarizer sheet, which enables intensity reduction as a result ofany change in the polarization state of the light as it passes throughthe EASLM.

Where an EASLM performs phase modulation, in a typical configuration theincident read optical beams will be linearly polarized by passing thebeams through a linear polarizer sheet, unless they are already in adefined linear polarization state. Phase modulation is controlled byapplication of an electric field, which influences the phase state ofthe light. In one example of phase modulation implemented using anematic phase liquid crystal, the optic axis direction is fixed in spacebut the birefringence is a function of the applied voltage. In oneexample of phase modulation implemented using a ferroelectric liquidcrystal, the birefringence is fixed, but the direction of the optic axisis controlled by the applied voltage. In phase modulation implementedusing either method, the output beam has a phase difference with respectto the input beam that is a function of the applied voltage. An exampleof a liquid crystal cell which can perform phase modulation is aFreedericksz cell arrangement in which anti-parallel aligned domains ofa nematic liquid crystal with a positive dielectric anisotropy are used,as described in U.S. Pat. No. 5,973,817.

A compact assembly for use in a compact holographic display comprisestwo EASLMs that are joined with a small or a minimal separation. In apreferred embodiment, both SLMs have the same number of pixels. Becausethe two EASLMs are not equidistant from the observer, the pixel pitch ofthe two EASLMs may need to be slightly different (but would still beapproximately the same) to compensate for the effect of being atdifferent distances with respect to observer. The light that has passedthrough a pixel of the first SLM passes through the corresponding pixelof the second SLM. Therefore, the light is modulated by both SLMs, andcomplex modulation of amplitude and phase independently can be achieved.As an example, the first SLM is amplitude-modulating and the second SLMis phase-modulating. Also, any other combination of modulationcharacteristics of the two SLMs is possible that together facilitatesindependent modulation of amplitude and phase.

Care has to be taken that light that has passed through a pixel of thefirst SLM passes only through the corresponding pixel of the second SLM.Crosstalk wilt occur if light from a pixel of the first SLM passesthrough non-corresponding, neighboring pixels of the second SLM. Thiscrosstalk may lead to a reduced image quality. Here are four possibleapproaches to the problem of minimizing the cross-talk between pixels.It will be apparent to those skilled in the art that these approachesmay also be applied to the implementation in section B.

(1) The first and simplest approach is to directly join or glue togethertwo SLMs, with aligned pixels. There will be diffraction at a pixel ofthe first SLM which causes a diverging propagation of light. Theseparation between the SLMs has to be thin enough such as to keep toacceptable levels the crosstalk between neighboring pixels of the secondSLM. As an example, with a pixel pitch of 10 μm the separation of thetwo EASLMs has to be less than or equal to the order of 10-100 μm. Thiscan hardly be achieved with conventionally manufactured SLMs, as thethickness of the cover glass is of the order of 1 mm. Rather, thesandwich is preferably manufactured in one process, with only a thinseparation layer between the SLMs. Manufacturing approaches outlined inthe section Outline Manufacturing Process may be applied to making adevice which includes two EASLMs separated by a small or minimaldistance.

FIG. 14 shows Fresnel diffraction profiles calculated for diffractionfrom a slit 10 μm wide, for various distances from the slit, in a twodimensional model, where the dimensions are perpendicular to the slit(z), and transverse to the slit (x). The slit of uniform illumination islocated between −5 μm and +5 μm on the x axis, with z equal to zeromicrons. The light transmitting medium is taken to have a refractiveindex of 1.5, which may be representative of media which would be usedin a compact device. The light was taken to be red light with a vacuumwavelength of 633 nm. Green and blue wavelengths have shorterwavelengths than red light, hence the calculations for red lightrepresent the strongest diffraction effects for the three colours red,green and blue. Calculations were performed using MathCad® software soldby Parametric Technology® Corp., Needham, Mass., USA. FIG. 15 shows thefraction of the intensity which remains within a 10 μm width centred onthe slit centre, as a function of distance from the slit. At a distanceof 20 μm from the slit, FIG. 15 shows that greater than 90% of theintensity is still within the 10 μm width of the slit. Hence less thanabout 5% of the pixel intensity would be incident on each adjacentpixel, in this two dimensional model. This calculation is in thelimiting case of zero boundary width between pixels. Real boundarywidths between pixels are greater than zero, hence for a real system thecross-talk problem would be lower than calculated here. In FIG. 14 theFresnel diffraction profiles close to the slit, such as at 50 μm fromthe slit, also approximate somewhat the top-hat intensity function atthe slit. Hence there are not broad diffraction features close to theslit. Broad diffraction features are characteristic of the far-fielddiffraction function of the top-hat function, which is a sinc squaredfunction, as known to those skilled in the art. Broad diffractionfeatures are observed in FIG. 14 for the case of a 300 μm distance fromthe slit. This shows that diffraction effects can be controlled byplacing the two EASLMs in close enough proximity, and that an advantageof placing the two EASLMs in close proximity is that the functional formof the diffraction profile changes from that characteristic of the farfield to a functional form which is more effective at containing thelight close to the axis perpendicular to the slit. This advantage is onewhich is counter to the mind set of those skilled in the art ofholography, as those skilled in the art tend to expect strong,significant and unavoidable diffraction effects when light passesthrough the small apertures of an SLM. Hence one skilled in the artwould not be motivated to place two SLMs close together, as one wouldexpect this to lead to inevitable and serious problems with pixelcross-talk due to diffraction effects.

FIG. 16 shows a contour plot of the intensity distribution as a functionof the distance from the slit. The contour lines are plotted on alogarithmic scale, not on a linear scale. Ten contour lines are used,which cover in total an intensity factor range of 100. The large degreeof confinement of the intensity distribution to the 10 μm slit width fordistances within about 50 μm from the slit is clear.

In a further embodiment, the aperture area of the pixels in the firstEASLM may be reduced to reduce cross-talk problems at the second EASLM.

(2) A second approach uses a lens array between the two SLMs, as shownin FIG. 17. Preferably, the number of lenses is the same as the numberof pixels in each SLM. The pitches of the two SLMs and of the lens arraymay be slightly different to compensate for the differences in thedistance from the observer. Each lens images a pixel of the first SLM onthe respective pixel of the second SLM, as shown by the bundle of light171 in FIG. 17. There will also be light through the neighboring lensthat may cause crosstalk, as shown by the bundle of light 172. This maybe neglected if either its intensity is sufficiently low or itsdirection is sufficiently different so that it does not reach the VOW.

The numerical aperture (NA) of each lens has to be sufficiently large inorder to image the pixel with sufficient resolution. As an example, fora resolution of 5 μm a NA 0.2 is required. This means that if geometricoptics is assumed, the maximum distance between the lens array and eachSLM is about 25 μm if the pitch of the SLM and the lens array is 10 μm.

It is also possible to assign several pixels of each SLM to one lens ofthe lens array. As an example, a group of four pixels of the first SLMmay be imaged to a group of four pixels of the second SLM by a lens ofthe lens array. The number of lenses of such a lens array would be afourth of the number of pixels in each SLM. This allows a higher NA ofthe lenses and hence higher resolution of the imaged pixels.

(3) A third approach is to reduce the aperture of the pixels of thefirst EASLM as much as possible. From a diffraction point of view, thearea of the second SLM that is illuminated by a pixel of the first SLMis determined by the aperture width D of the pixel of the first EASLMand by the diffraction angle, as shown in FIG. 18. In FIG. 18, d is thedistance between the two EASLMs, and w is the distance between the twofirst order diffraction minima which occur either side of the zero ordermaximum. This is assuming Fraunhofer diffraction, or a reasonableapproximation to Fraunhofer diffraction.

Reducing the aperture width D on the one hand reduces the directlyprojected area in the central part of the illuminated area, as indicatedby the dotted lines in FIG. 18. On the other hand, the diffraction angleis increased, as the diffraction angle is proportional to 1/D inFraunhofer diffraction. This increases the width w of the illuminatedarea on the second EASLM. The illuminated area has the total width w. Ina Fraunhofer diffraction regime, D may be determined such that itminimizes w at a given separation d, using the equation w=D+2dλ/D whichis derived from the distance between the two first order minima inFraunhofer diffraction.

For example, if λ is 0.5 μm, d is 100 μm and w is 20 μm, one obtains aminimum in D for D of 10 μm. While the Fraunhofer regime may not be agood approximation in this example, this example illustrates theprinciple of using the distance between the EASLMs to control thediffraction process in the Fraunhofer diffraction regime.

(4) A fourth approach uses a fiber optic faceplate to image the pixelsof the first SLM on the pixels of the second SLM. A fiber opticfaceplate consists of a 2D arrangement of parallel optic fibers. Thelength of the fibers and hence the thickness of the faceplate istypically several millimeters and the length of the diagonal across theface of the plate is up to several inches. As an example, the pitch ofthe fibers may be 6 μm. Fibre optic faceplates with such a fibre pitchare sold by Edmund Optics Inc. of Barrington, N.J., USA. Each fiberguides light from one of its ends to the other end. Therefore, an imageon one side of the faceplate is transferred to the other side, with highresolution and without focusing elements. Such a faceplate may be usedas a separating layer between the two SLMs, as shown in FIG. 19.Multimode fibres are preferred over single mode fibres, becausemultimode fibres have better coupling efficiency than single modefibres. Coupling efficiency is optimal when the refractive index of thecore of the fibre is matched to the refractive index of the liquidcrystal, as this minimizes Fresnel back reflection losses.

There are no additional cover glasses between the two SLMs. Thepolarizer, the electrodes and the alignment layers are directly attachedto the fiber optic faceplate. Each of these layers is very thin, i.e. ofthe order of 1-10 μm. Therefore, the liquid crystal (LC) layers LC1 andLC2 are in close vicinity to the faceplate. The light that has passedthrough a pixel of the first SLM is guided to the respective pixel ofthe second SLM. This minimizes crosstalk to the neighboring pixels. Thefaceplate transfers the light distribution at the output of the firstSLM to the input of the second SLM. On average there should be at leastone fibre per pixel. If there is less than one fibre per pixel, onaverage, SLM resolution will be lost, which will reduce the quality ofthe image shown in an application in a holographic display.

In FIG. 19, the first SLM modulates the amplitude and the second SLMmodulates the phase. Other modulation characteristics are possible forthe two EASLMs that in combination facilitate a full complex modulation.

An example of a compact arrangement for encoding amplitude and phaseinformation in a hologram is disclosed in FIG. 10. 104 is anillumination apparatus for providing illumination of a plane area, wherethe illumination has sufficient coherence so as to be able to lead tothe generation of a three dimensional image. An example of anillumination apparatus is disclosed in US 2006/250671 for the case oflarge area video holograms. Such an apparatus as 104 may take the formof an array of a white light sources, such as cold cathode fluorescentlamps or white light light emitting diodes which emit light which isincident on a focusing system which may be compact such as a lenticulararray or a microlens array 100. Alternatively, light sources for 104 maycomprise of red, green and blue lasers or red, green and blue lightemitting diodes which emit light of sufficient coherence. However,non-laser sources with sufficient spatial coherence (eg. light emittingdiodes, OLEDs, cold cathode fluorescent lamps) are preferred to lasersources. Laser sources have disadvantages such as causing laser specklein the holographic reconstructions, being relatively expensive, andhaving possible safety problems with regard to possibly damaging theeyes of holographic display viewers or of those who work in assemblingthe holographic display devices.

Element 104 may include one or two prismatic optical films forincreasing display brightness: such films are disclosed eg. in U.S. Pat.No. 5,056,892 and in U.S. Pat. No. 5,919,551, though others are known.Element 104 may include a polarizing optical element, or a set ofpolarizing optical elements. One example is a linear polarizer sheet. Afurther example is a reflective polarizer which transmits one linearpolarization state and reflects the orthogonal linear polarizationstate—such a sheet is described in U.S. Pat. No. 5,828,488, for example,though others are known. A further example is a reflective polarizerwhich transmits one circular polarization state and reflects theorthogonal circular polarization state—such a sheet is described in U.S.Pat. No. 6,181,395, for example, though others are known. Element 104may include other optical elements which are known in the field ofbacklight technology.

Elements 104, 100-103 may be about a few centimetres in thickness, orless, in total. Element 101 may comprise of an array of colour filters,such that pixels of colour light, such as red, green and blue light, areemitted towards element 102, although the colour filters may not berequired if coloured sources of light are used. Element 102 is an EASLMwhich encodes phase information, such as a Freedericksz cell. Element103 is an EASLM which encodes amplitude information, such as in aconventional commercially available liquid crystal display device. Eachcell in element 102, represented here by 107, is aligned with acorresponding cell in element 103, represented here by 108. However,although the cells in elements 102 and 103 have the same lateralspacing, or pitch, the cells in element 102 are smaller than or the samesize as the cells in element 103, because light exiting cell 107 maytypically undergo some diffraction before entering cell 108 in element103. The order in which amplitude and phase are encoded may be reversedfrom that shown in FIG. 10.

A viewer located at point 106 some distance from the device whichincludes the compact hologram generator 105 may view a three dimensionalimage when viewing in the direction of 105. Elements 104, 100, 101, 102and 103 are arranged so as to be in physical contact as described above,so as to form a compact hologram generator 105.

E. Large Magnification Three Dimensional Image Display Device ComponentIncorporating the Compact Combination of One or Two Pairs of OLED andOASLM Combinations, or One or Two EASLMs, with HolographicReconstruction of the Object

A large magnification three dimensional image display device componentincorporating the compact combination of one or two pairs of OLED andOASLM combinations, or one or two EASLMs, with holographicreconstruction of the object, is shown in FIG. 24. The device componentincludes a compact combination of an SLM and a compact light source ofsufficient coherence (such as those described in sections A, B, C and Dabove), the combination being capable of generating a three dimensionalimage viewable in a VOW (denoted OW in FIG. 24) under suitableillumination conditions, where the device component may be incorporatedin a PDA or in a mobile phone, for example. The compact combination ofan SLM and a compact light source of sufficient coherence comprises anarray of light sources, an SLM and a lens array, as shown in FIG. 24.The SLM in FIG. 24 incorporates the compact combination of one or twopairs of OLED and OASLM combinations, or one or two EASLMs, or onecombination pair of an OLED and an OASLM, and one EASLM.

In a simple example, an array of light sources may be formed as follows.A single light source such as a monochromatic LED is placed next to anarray of apertures such that the apertures are illuminated. If theapertures are a one dimensional array of slits, the light transmitted bythe slits forms a one dimensional array of light sources. If theapertures are a two dimensional array of circles, the illuminated set ofcircles forms a two dimensional array of light sources. A typicalaperture width will be about 20 μm. Such an array of light sources issuitable for contributing to the generation of a VW for one eye.

In FIG. 24, the array of light sources is situated at a distance u fromthe lens array. The array of light sources may be the light sources ofelement 10 of FIG. 1, and may optionally incorporate element 11 ofFIG. 1. To be precise, each source of light in the light source array issituated at a distance u from its corresponding lens in the lens array.The planes of the light source array and of the lens array are parallelin a preferred embodiment. The SLM may be located at either side of thelens array. The VOW is at a distance u from the lens array. The lensesin the lens array are converging lenses with a focal length f given byf=1/[1/u+1/v]. In a preferred embodiment, v is in the range of 300 mm to600 mm. In a particularly preferred embodiment v is about 400 mm. In apreferred embodiment u is in the range of 10 mm to 30 mm. In aparticularly preferred embodiment u is about 20 mm. The magnificationfactor M is given by v/u. M is the factor by which the light sources,which have been modulated by the SLM, are magnified at the VOW. In apreferred embodiment, M is in the range of 10 to 60. In a particularlypreferred embodiment, M is about 20. To achieve such magnificationfactors with good holographic image quality requires accurate alignmentof the light source array and the lens array. Significant mechanicalstability of the device component is required, in order to maintain thisaccurate alignment, and to maintain the same distance between the lightsource array and the lens array, over the operating lifetime of thecomponent.

The VOW may be trackable or non-trackable. If the VOW is trackable, thendepending on the required position of the VOW, specific light sources inthe array of light sources are activated. The activated light sourcesilluminate the SLM and are imaged into the observer plane by the lensarray. At least one light source per lens of the lens array is activatedin the light source array. The tracking is quasi-continuous. If u is 20mm and v is 400 mm, the VOW can be tracked with a lateral increment of400 μm if the pixel pitch is 20 μm. This tracking is quasi-continuous.If u is 20 mm and v is 400 mm, f is approximately 19 mm.

The light sources in the light source array may have only partialspatial coherence. Partial coherence leads to a smeared reconstructionof the object points. If u is 20 mm and v is 400 mm, an object point ata distance of 100 mm from the display is reconstructed with a lateralsmearing of 100 μm if the light source width is 20 μm. This issufficient for the resolution of the human vision system.

There does not have to be any significant mutual coherence between thelight that passes through different lenses of the lens array. Thecoherence requirement is limited to each single lens of the lens array.Therefore, the resolution of a reconstructed object point is determinedby the pitch of the lens array. A typical lens pitch will be of theorder of 1 mm to guarantee sufficient resolution for the human visionsystem.

The VOW is limited to one diffraction order of the Fourier spectrum ofthe information encoded in the SLM. At a wavelength of 500 nm the VOWhas a width of 10 mm if the pixel pitch of the SLM is 10 μm and twopixels are needed to encode one complex number i.e. if 2-phase encodingon a phase-modulating EASLM is used. The VOW may be enlarged by tilingof VOWs by spatial or temporal multiplexing. In the case of spatialmultiplexing additional optical elements such as beam splitters arerequired. Some approaches to multiplexing which may also be employed inthis implementation are described in section C above.

Color holographic reconstructions can be achieved by temporalmultiplexing. The red, green and blue pixels of a color OLED display aresequentially activated with synchronous re-encoding of the SLM withholograms calculated for red, green and blue optical wavelengths.

The display of which the device component forms a part may comprise aneye position detector that detects the positions of the observer's eyes.The eye position detector is connected with a control unit that controlsthe activation of the light sources within the array of light sources.

The calculation of the holograms that are encoded on the SLM ispreferably performed in an external encoding unit as it requires highcomputational power. The display data are then sent to the PDA or mobilephone to enable the display of a holographically-generated threedimensional image.

As a practical example, a 2.6 inch screen diagonal XGA LCD EASLM made bySanyo® Epson® Imaging Devices Corporation of Japan may be used. Thesubpixel pitch is 17 μm. If this is used in constructing an RGBholographic display, with amplitude modulation encoding of the hologram,at a distance of 0.4 m from the EASLM the viewing window is calculatedto be 1.3 mm across. For the monochrome case, the viewing window iscalculated to be 4 mm across. If the same configuration is used, but itis implemented using phase modulation with two-phase encoding, theviewing window is calculated to be 6 mm across. If the sameconfiguration is used, but it is implemented using phase modulation withKinoform encoding, the viewing window is calculated to be 12 mm across.

Other high resolution EASLM examples exist. Seiko® Epson® Corporation ofJapan has released monochrome EASLMs, such as the D4:L3D13U1.3 inchscreen diagonal panel with a pixel pitch of 15 μm. The same company hasreleased a D5: L3D09U-61G00 panel in the same panel family with a screendiagonal length of 0.9 inches and a pixel pitch of 10 μm. On Dec. 12,2006 the same company announced the release of a L3D07U-81G00 panel inthe same family with a screen diagonal length of 0.7 inches and a pixelpitch of 8.5 μm. If the D4:L3D13U1.3 inch panel is used in constructinga monochrome holographic display, with Burckhardt amplitude modulationencoding of the hologram, at a distance of 0.4 m from the EASLM the VWis calculated to be 5.6 mm across.

F. Three Dimensional Image Display Device Incorporating the CompactCombination of One or Two Pairs of OLED and OASLM Combinations, or Oneor Two EASLMs, with Holographic Reconstruction of the Object

The compact combination of one or two pairs of OLED and OASLMcombinations, or one or two EASLMs, preferably can be used in ahand-held three dimensional display device, or in a larger threedimensional display device, as the combination can be very compact. Thecombination may be integrated in a mobile phone, a satellite navigationdevice, an automotive display, a computer game device, a personaldigital assistant (PDA), a laptop computer display, a desktop computermonitor, or a slim television display, for example. Such a threedimensional display is preferably for a single user only. The user islocated at a position generally perpendicular to the device's lightemitting surface and at a distance from the device from which optimalviewing is achieved such as a distance of approximately 500 mm. It isknown that a user of a hand-held device will tend automatically toorient the device in the hand so as to achieve the optimum viewingconditions, as described for example in WO01/96941. Therefore, in suchdevices there is no necessity for user eye tracking and for complicatedand non-compact tracking optics comprising scanning mirrors, forexample. But eye tracking could be implemented for such devices if theadditional requirements for apparatus and electrical power do not imposean excessive burden.

The benefits of a satellite navigation three dimensional image displaydevice incorporating the compact combination of one or two pairs of OLEDand OASLM combinations, or one or two EASLMs, with holographicreconstruction of the object include the following. The driver may finda three dimensional image of route information, such as the maneuver tobe made at the next intersection, preferable to two dimensional imageinformation, as three dimensional image information corresponds moreclosely to what the driver perceives while driving. Other information onthe display, such as menu icons, may be displayed three dimensionally.Some or all information on the display may be displayed threedimensionally.

The benefits of an automotive three dimensional image display deviceincorporating the compact combination of one or two pairs of OLED andOASLM combinations, or one or two EASLMs, with holographicreconstruction of the object, include the following. The device may beable to display three dimensional information directly, such as a threedimensional image of the car's bumper (fender) in proximity to an objectnear to the vehicle, such as a wall, during a reversing maneuver, orwhile attempting to drive through an opening not much wider than thevehicle, or narrower than the vehicle. Where the opening is narrowerthan the vehicle, the three dimensional image display device may helpthe driver to realize that the vehicle wilt not go through the opening.The three dimensional image could be constructed using information fromsensors mounted within or on the vehicle. Other vehicle informationcould be displayed three dimensionally on the display, such as speed,temperature, engine revolutions per minute, or other information knownto be displayed within vehicles. Satellite navigation information may bedisplayed three dimensionally on the display. Some or all information onthe display may be displayed three dimensionally.

The size of the output window is limited by the periodicity interval ofthe diffraction pattern in the Fourier plane. If the pixel pitch in theOLED-display, or in the EASLM, is approximately 10 μm then for visiblelight of wavelength 500 nm the virtual observer window (VOW) width isapproximately 10 mm to 25 mm at distance of 500 mm, depending on theencoding used in the SLM for the hologram. This is sufficient width forone eye. A second VOW for the other eye may be created by spatial ortime multiplexing of the content of the spatial light modulators. In theabsence of tracking, in order to see the optimum three dimensionalimage, the observer has to orient and to move the device and/or himselfso that his eyes are in the VOWs and at the optimum distance from thedevice.

The process of adjusting the position and orientation of the displaydevice can be made easier by tiling several VOWs. Two or three VOWs maybe juxtaposed in the x- and y-directions so that a larger area may becovered by the VOWs. Tiling can be performed by spatial or timemultiplexing, or by a combination of spatial and time multiplexing.

In time multiplexing, the light is projected time-sequentially into theVOWs. The spatial light modulators have to be re-encoded if the VOWshave differing content. In spatial multiplexing, the content for thedifferent VOWs are encoded in the spatial light modulators at the sametime, but in different areas of the spatial light modulators. A beamsplitter may split the light from the different areas of the spatiallight modulators to different VOWs. A combination of spatial and timemultiplexing can be used.

The typical screen diagonal size of the hand-held three dimensionaldisplay for use in a mobile phone or a PDA is in the range from one inchto several inches. A holographic sub-display could have a screendiagonal as small as one cm.

The three dimensional image display may be switched to display twodimensional images, such as by displaying identical images to each eyeof a viewer's two eyes.

An implementation of the three dimensional image display deviceincorporating the compact combination of one or two pairs of OLED andOASLM combinations, or one or two EASLMs, is shown in FIG. 3. The devicein FIG. 3 is a mobile phone 30 on which one may make a telephone callduring which a three dimensional video image of the other party,suitably equipped with a similar device, is displayed in the screenregion 31. The mobile phone is equipped with an antenna 32 for mobilecommunication. In other embodiments, the antenna may be within the bodyof the mobile phone 30. The mobile phone 30 is equipped with two cameras33 and 34 which record right eye and left eye views of the user,respectively. The right eye and left eye views comprise stereoscopicimage data. The mobile phone 30 is equipped with keys 35 for numeralsand for the “*” and “#” symbols, and keys 36 for other functions such asmoving within on-screen menus, backspacing or turning the unit on oroff. Labels present on the keys such as “ON” “OFF” or “2” are such thatthey prevent the unit from being used upside down, which prevents bothparties in the three dimensional video telephone call from viewing theother party upside down. In use, preferably the two viewer's eyes andthe two cameras 33 and 34 are coplanar, and the user's face is locatedapproximately perpendicular to the screen region 31. This ensures thatthe two cameras 33 and 34 record parallax in the plane which containsthe viewer's eyes. The optimum viewing position for the viewer's headwith respect to the display is predetermined such that the two cameras33 and 34 obtain optimum image quality of the viewer's head in thisposition. The same being true of the counterparty in a three dimensionalvideo telephone call, the two parties may engage in a two-way threedimensional video telephone call with optimum image quality. To ensurethat each viewer points the cameras 33 and 34 at their own faceaccurately, it may be desirable to ensure that the virtual observerwindows for each eye are not much bigger than each eye, as this willlimit the viewer's scope for positional and orientational error of theircamera directions. By pointing the device at an object to bephotographed, the device may take three dimensional photographs of anobject. Alternatively, the user could be guided to achieve the optimumorientation of the device through the use of small icons on the devicescreen. The device may also implement eye tracking. The device formatand usage described here may be used for a device which generates threedimensional images holographically, autostereoscopically or by any otherapproaches.

During a two-way three dimensional video telephone call, the cameras 33and 34 record right eye and left eye views of a user, respectively. Thedata obtained from these views is used to construct the threedimensional video image on the corresponding device held by thecounterparty in the three dimensional video telephone call. If the threedimensional image is generated autostereoscopically, the views from thecameras 33 and 34 may be used directly in generating the two eye imagesin the autostereoscopic display. If the three dimensional image isgenerated holographically, the data comprising the views from thecameras 33 and 34 should be processed such as to permit suitableencoding of the holographic data onto one or two SLMs, such as byutilizing a computer generated hologram. When the three dimensionalimage is generated holographically, the three dimensional display is aholographic display. A holographic display provides full depthinformation, i.e. accomodation (eye focusing) and parallax, in contrastto an autostereoscopic display. A holographic display gives aholographic reconstruction of an object, i.e. holographic reconstructionof all object points at the correct depth.

An application of the hand held three dimensional display described hereincludes holding a two-way three dimensional video telephone call. Afurther application includes being shown a three dimensional view of anobject or a scene by the counterparty in the telephone call, eg. to viewan item prior to purchase, or to inspect an object for damage. A furtherapplication includes confirming the identity of an individual, which maybe facilitated by a three dimensional view. The ability to distinguishbetween individuals very similar in appearance, such as twins, or aperson in disguise, may be facilitated by a three dimensional view. Afurther application includes viewing an individual with a view to makingfurther contact, such as within a dating service, where the decision maybe facilitated by a three dimensional view. A further applicationincludes the activity of viewing adult content with a three dimensionalview, where a viewer may prefer a three dimensional view to a twodimensional view.

Different individuals have different distances between their eyes. Inone implementation, the three dimensional display device withholographic reconstruction of the object has a menu option which enablesthe user of the display to vary the distance between the projected lefteye and right eye virtual observer windows. On selection of the menuoption, the user presses keys on the device key pad to either increase,or to decrease, the separation between the virtual observer windows. Ifthis is done while looking at the display and seeking to view a threedimensional image, the separation between the virtual observer windowsmay be selected which gives the viewer the best three dimensional imagethat they can perceive. The selected distance may then be saved as auser preference. Multiple user preferences may be saved on the device,if the device is to be used by more than one individual. Such a menuoption may be implemented even if the device has the capability to trackthe positions of the obsever's eyes independently, as a user may bebetter than the tracking software at selecting the precise distance theyprefer between the virtual observer windows. Once such a selection hasbeen made, this may speed up tracking, as a less accurate positiondetermination may be required for the observer's eyes after the distancebetween the eyes becomes a fixed parameter. Being able to select apreferred distance between the two virtual observer windows also offersan advantage over autostereoscopic systems, in which the distancebetween the left eye and right eye views tends to be fixed by the devicehardware.

G. 2D-Projector which Incorporates the Compact Combination of One or TwoPairs of OLED and OASLM Combinations, or One or Two EASLMs

Instead of projecting the light into a number of VOWs, as in section Fabove, the light from the device may also be projected onto a screen ora wall or some other surface. Thus the three dimensional display devicein a mobile phone or PDA or in some other device can also be used as apocket projector.

An improved quality of holographic projection may be obtained by using aSLM that modulates the amplitude and the phase of the incident light.Thus a complex-valued hologram can be encoded on the SLM, which mayresult in a better quality of the image reconstructed on the screen orwall.

The compact combination of one or two pairs of OLED and OASLMcombinations, or one or two EASLMs, described in the previous sections,can be used as a SLM in a projector. Due to the compact size of thecombination, the projector will also be compact. The projector can evenbe the same device as the mobile phone or the PDA or some other device:it may be switched between the modes “three dimensional display” and“projector”.

Compared to conventional 2D projectors, a holographic 2D projector hasthe advantage that no projection lenses are needed and that theprojected image is focused at all distances in the optical far field.Prior art holographic 2D projectors, such as disclosed in WO2005/059881,use a single SLM that is therefore not capable of complex modulation.The holographic 2D projector disclosed here would be capable of complexmodulation and would therefore have superior image quality.

H. Autostereoscopic or Holographic Display Using One or Two CompactCombinations of an Infra Red OLED Display and OASLM

The compact combination of an infra red OLED display and OASLM (eg.described in section A above) can also be used in an autostereoscopicdisplay (ASD), preferably a hand-held ASD in a mobile phone or a PDA.Whereas for a typical viewer an ASD may not be as comfortable to view asa holographic display, as ASD may be cheaper or easier to fabricate orto supply image data to than a holographic display in somecircumstances. An ASD provides several viewing zones, whereby eachviewing zone shows a different perspective view of the 3D-scene. Theobserver sees a stereoscopic image if his eyes are in different viewingzones. Note the difference between ASD and holography: ASD provides twoflat images whereas holography also provides the z-information of eachobject point in the 3D-scene.

Usually an ASD is based on spatial multiplexing of the viewing zones onthe display and using beam splitter elements, e.g. lenticulars, barriermasks or prism masks. The barrier masks may also be referred to as“parallax barriers.” As a disadvantage, for an ASD the resolution ineach viewing zone typically is reduced in inverse proportion to thenumber of viewing zones. But this disadvantage may be offset by theadvantages that an ASD may possess, as described above.

The compact combination of an infra red OLED display and anamplitude-modulating OASLM (eg. described in section A above) can beused as an amplitude-modulating display with high resolution. Ahigh-resolution ASD can be constructed if the compact combination of aninfra red OLED display and an amplitude-modulating OASLM is combinedwith beam splitter elements. The high-resolution of the compactcombination may offset the loss of resolution due to spatialmultiplexing.

An advantage of using the compact combination of one or more OLED arraysand one or more OASLMs (eg. described in sections A and B above) for anASD, which necessitates one or more additional OASLM components, is thenon-patterned OASLM. An ASD comprising a beam splitter and an OLED arraymay have artefacts due to the patterned OLED, e.g. Moiré effects betweenthe period of the beam splitter and the period of the OLED. In contrastthereto, the information on the OASLM of the compact combination iscontinuous: there is only the period of the beamsplitter, and theperiod-based artefacts do not occur.

The ASD light source may be one or more light sources, such as LEDs,lasers, OLEDs or CCFLs. The light sources need not be coherent. If whitelight sources are used, a layer of colour filters, such as red, greenand blue filters, will be required between the light source and thecompact combination of a light emitting display and anamplitude-modulating OASLM, if the ASD is to display colour images.

The compact combination of an infra red OLED display and OASLM (eg.described in section A above) can also be used in a holographic display,preferably a hand-held display in a mobile phone or a PDA. Here theholographic display is based on spatial multiplexing of the viewingzones on the display and using beam splitter elements, e.g. lenticulars,barrier masks or prism masks. The barrier masks may also be referred toas “parallax barriers.” The compact combination of an infra red OLEDdisplay and an OASLM (eg. described in section A above), can be used asa holographic display with high resolution. A high-resolutionholographic display can be constructed if the compact combination of aninfra red OLED display and an amplitude-modulating OASLM is combinedwith beam splitter elements. The high-resolution of the compactcombination may offset the loss of resolution due to spatialmultiplexing. In a further implementation, a combination of two pairs ofa compact combination of an OLED array and an OASLM can be used tomodulate the amplitude and the phase of light in sequence and in acompact way, as described in section B. Thus, a complex number, whichconsists of an amplitude and a phase, can be encoded in the transmittedlight, on a pixel by pixel basis. A high-resolution holographic displaycan be constructed if the compact combination of two pairs of an infrared OLED display and an amplitude-modulating OASLM is combined with beamsplitter elements. The high-resolution of the compact combination mayoffset the loss of resolution due to spatial multiplexing. A holographicdisplay with beam splitter elements may provide several viewing zones,whereby each viewing zone shows a different perspective view of the3D-scene. The observer sees a holographic image if his eyes are indifferent viewing zones.

I. Data Processing System Required in Three Dimensional Communication

The data processing system required in three dimensional communicationis shown schematically in FIG. 22. In FIG. 22, a party 220 is in threedimensional communication with another party 221. Camera data for use inconstructing an image may be collected using a mobile phone device 30shown in FIG. 3, or by some device with a similar function. Dataprocessing for three dimensional image display may be performed in thedevice of party 220 which may be a mobile phone 30 or an equivalentdevice, or it may be performed in the device of the other party 221, butpreferably it is performed at an intermediate system 224 located on thetransmission network between the two mobile phones. The transmissionnetwork comprises a first link 222, an intermediate system 224, and asecond link 223. The two links 222 and 223 may be wireless links ornon-wireless links. The intermediate system 224 may include a computerfor performing calculations to enable the display of three dimensionalimages, such as computer generated holograms, or autostereoscopicimages. The use of a computer in the transmission network between thetwo mobile phones to perform the calculations is preferable, as thecomputation will not use up mobile phone battery power, but may insteaduse mains electricity power. The computer located on the transmissionnetwork may be used to perform the image processing for a large numberof three dimensional video telephone calls simultaneously, which maypermit more efficient use of computation resources, such as by reducingthe amount of unused computational processing power. The weight of themobile phone or equivalent device will be reduced if its requirementsfor computational processing power are reduced, because it will requireless computer circuitry and memory and because the computationallydemanding calculations will be performed by a computer located on thetransmission network. Finally, the software which performs thecalculations will only need to be installed on the computer located onthe transmission network and not on the mobile phone or equivalentdevice. This will reduce the mobile phone memory requirements, the scopefor software piracy and it will improve the protection of any industrialsecrets present in the software code. While the bulk of the calculationsrequired for three dimensional image display may be performed by theintermediate system 224, it is possible that some image calculations maybe performed by the user device prior to data transmission. If the twocamera images are quite similar, data transmission may be facilitated ifthe two images are sent as a first image and as a difference image,where the difference image is the difference between the two images, asthe difference image may lend itself more readily to data compressiontechniques which facilitate data transmission, for example. Likewise,the three dimensional image display device may perform some imagecalculations, such as decompressing compressed image data.

In one example of the system of FIG. 22, a first image and a secondimage which form a pair of stereoscopic images, are sent by the deviceof user 220 via link 222 to the intermediary device 224. The secondtransmitted image may be the difference image between the twostereoscopic images, as a difference image will typically require lessdata than a complete image. If a three dimensional telephoneconversation is in progress, the first image may itself be expressed asthe difference between the present image and the image from one timestepearlier. Similarly the second image may be expressed as the differencebetween the present image and the image from one timestep earlier. Theintermediary device 224 may then calculate a two dimensional (2D) image,with its corresponding depth map, from the data received, usingcalculation procedures for converting between 2D and three dimensional(3D) images known in the art. In the case of a colour image, threecomponent 2D images in the three primary colours are required, togetherwith their corresponding depth maps. The data corresponding to the 2Dimages and depth maps may then be transmitted to the device of user 221via link 223. The device of user 221 encodes the holograms in itscompact three dimensional display device based on the 2D images anddepth maps received. To make efficient use of transmission bandwidth,the data transmitted within this system may be subjected to knowncompression procedures, with corresponding decompression being performedby the receiving device. The most efficient amount of data compressionto be used balances the power required from the battery of the mobiledevice in performing data compression or decompression against the costof the bandwidth required if less data compression is used.

The intermediary device 224 may have access to a library containing aset of known 3D shapes, to which it may try to match its calculated 3Ddata, or it may have access to a library containing a set of known 2Dprofiles to which it may try to match incoming 2D image data. If a goodmatch can be found with respect to a known shape, this may speed upcalculation processes, as 2D or 3D images may then be expressed relativeto a known shape. Libraries of 3D shapes may be provided such as theface or body shapes of a set of sports stars such as leading tennisplayers or soccer players, and the shapes of all or parts of leadingsports venues such as famous tennis courts or famous soccer grounds. Forexample, a 3D image of a person's face may be expressed as being one towhich intermediary device 224 has access, plus a change to the facialexpression which may be a smile or a frown for example, plus some changein the hair length as the hair may have grown or been cut since thestored data was obtained, for example. The data to which theintermediary device 224 has access may be updated by intermediary device224 if a persistent set of differences emerges such that it is clearthat the data to which the intermediary device 224 has access has becomeout of date, eg. the person's hair length has been changed significantlyand on a long term basis. If the intermediary device 224 encounters a 2Dor 3D image to which no good match can be found in the records to whichit has access, it may add the new shape to the set of records.

J. System for Boosting 2D Image Content to 3D Image Content

One difficulty in securing widespread adoption of three dimensionaldisplay technology is the fact that historically very little content hasbeen generated in a three dimensional format, and at present mostcontent continues to be generated in two dimensional format. This ispartly because most image recording apparatus in use at presentcontinues to record two dimensional images and not data which can beused in three dimensional image display. In addition, there arecurrently limited opportunities for a viewer to demand 3D content or toobtain 3D content which has been generated from 2D content.

There is clearly a need for a system which supports the generation ofthree dimensional content from two dimensional content. One system isgiven in FIG. 23, but others wilt be obvious to those skilled in theart. In FIG. 23, a TV broadcasting company 2300 continues to broadcasttwo dimensional TV images 2304, even though three dimensional displayapparatus is present in the home of the viewer 2302. In this system, anintermediary system 2301 is present which has the capability ofconverting 2D content to 3D content 2305. This conversion process may besupported by fees paid by the viewer, or it may be supported by fees byother parties, such as advertiser 2303. In FIG. 23, when theadvertisement of advertiser 2303 is broadcast by TV company 2300,advertiser 2303 pays a fee 2306 to intermediary system 2301 to convertthe 2D content to 3D content using known processes for converting 2Dcontent to 3D content. The advertiser benefits through a 3D TVadvertisement being shown to a viewer 2302, which may be moreeye-catching than a 2D TV advertisement. Alternatively, the viewer 2302may pay a fee to intermediary 2301 to convert some or all of the TVbroadcasts he receives to 3D format. The intermediary system ensuresthat the 3D content is provided in a properly synchronized format, suchas ensuring that for example if a 2D image is supplied with itscorresponding depth map, the two data sets are provided in asynchronized fashion i.e. that the 3D display device uses the depth mapfor the corresponding 2D image and not for a non-corresponding 2D image.The 3D display device may be a holographic display device, anautostereoscopic display device, or any known 3D display device. Thedata provided to the 3D display device should be appropriate for thattype of 3D display device. A similar system to the above may also beapplied to content provided by a provider other than a TV broadcastingcompany, such as a supplier of films (movies), videos or the like.

In an alternative system, the viewer may supply 2D content to theintermediary system, pay a fee, and receive in return a 3D version ofthe 2D content supplied. The 2D content supplied could be a MP3 file ofa home movie, for example, or other video content or image content suchas photographs or pictures.

The intermediary system may include a computer for performingcalculations to enable the display of three dimensional images, such ascomputer generated holograms, or autostereoscopic images. The use of acomputer in the transmission network between the 2D content supplier andthe viewer who wishes to view 3D content to perform the calculations ispreferable, as it may be more efficient than performing such processesin the location of the viewer. The computer located on the transmissionnetwork may be used to perform the image processing for a large numberof 2D to 3D content conversions simultaneously, which may permit moreefficient use of computation resources, such as by reducing the amountof unused computational processing power. The cost of the viewer's 3Ddisplay device will be reduced if its requirements for computationalprocessing power are reduced, because it will require less computercircuitry and memory and because the computationally demandingcalculations will be performed by a computer located on the transmissionnetwork. Finally, the software which performs the calculations will onlyneed to be installed on the computer located on the transmission networkand not on the viewer's 3D display device. This will reduce the viewer's3D display device memory requirements, the scope for software piracy andit will improve the protection of any industrial secrets present in thesoftware code. While the bulk of the calculations required for threedimensional image display may be performed by the intermediate system,it is possible that some image calculations may be performed by theviewer's 3D display device. The three dimensional image display devicemay perform some image calculations, such as decompressing compressedimage data, or generating holographic encoding of spatial lightmodulators from a 2D image and its corresponding depth map.

In one example, the intermediary may calculate a depth map whichcorresponds to a given 2D image, from the 2D image data received, usingcalculation procedures for converting between 2D and 3D images known inthe art. In the case of a colour image, three component 2D images in thethree primary colours are required, together with their correspondingdepth maps. The data corresponding to the 2D images and depth maps maythen be transmitted to the viewer's 3D display device. The viewer's 3Ddisplay device encodes the holograms in its spatial light modulatorsbased on the 2D images and depth maps received. To make efficient use oftransmission bandwidth, the data transmitted within this system may besubjected to known compression procedures, with correspondingdecompression being performed by the receiving device. The mostefficient amount of data compression to be used balances the cost ofproviding data decompression functionality to the 3D display deviceagainst the cost of the bandwidth required if less data compression isused.

The intermediary may have access to data about a set of known 3D shapes,to which it may try to match its calculated 3D data, or it may haveaccess to a set of known 2D profiles to which it may try to matchincoming 2D image data. If a good match can be found with respect to aknown shape, this may speed up calculation processes, as 2D or 3D imagesmay then be expressed relative to a known shape. Libraries of 3D shapesmay be provided such as the face or body shapes of a set or sports starssuch as leading tennis players or soccer players, and the shapes of allor parts of leading sports venues such as famous tennis courts or famoussoccer grounds. For example, a 3D image of a person's face may beexpressed as being one to which the intermediary has access, plus achange to the facial expression which may be a smile or a frown forexample, plus some change in the hair length as the hair may have grownor been cut since the stored data was obtained, for example. The data towhich the intermediary has access may be updated by the intermediary ifa persistent set of differences emerges such that it is clear that thedata to which the intermediary has access has become out of date, eg.the person's hair length has been changed significantly and on a longterm basis. If the intermediary encounters a 2D image to which no goodmatch can be found in the records to which it has access, it may add thenew calculated 3D shape to the set of records.

K. Spatial Multiplexing of Observer Windows and 2D-Encoding

This implementation relates to spatial multiplexing of virtual observerwindows (VOWs) of a holographic display combined with using 2D-encoding.Otherwise, the holographic display may be as described in sections A, B,C or D, or it may be any known holographic display.

It is known that several VOWs, e.g. one VOW for the left eye and one VOWfor the right eye, can be generated by spatial or temporal multiplexing.For spatial multiplexing, both VOWs are generated at the same time andare separated by a beam splitter, similar to an autostereoscopicdisplay, as described in WO 2006/027228, which is incorporated herein byreference. For temporal multiplexing, the VOWs are generated timesequentially.

However, known holographic display systems suffer some disadvantages.For spatial multiplexing an illumination system has been used that isspatially incoherent in the horizontal direction and which is based onhorizontal line light sources and a lenticular array, as shown forexample in prior art FIG. 4, which is taken from WO 2006/027228. Thishas the advantage that the techniques known from autostereoscopicdisplays can be used. However, there is the disadvantage that aholographic reconstruction in the horizontal direction is not possible.Instead, a so-called 1D-encoding is used that leads to holographicreconstruction and motion parallax only in the vertical direction.Hence, the vertical focal point is in the plane of the reconstructedobject, whereas the horizontal focal point is in the plane of the SLM.This astigmatism reduces the quality of spatial vision i.e. it reducesthe quality of the holographic reconstruction which is perceived by aviewer. Similarly, temporal multiplexing systems suffer a disadvantagein that they require fast SLMs which are not yet available in alldisplay sizes, and which even if available may be prohibitivelyexpensive.

Only 2D-encoding provides holographic reconstruction simultaneously inthe horizontal and the vertical directions and hence 2D-encodingproduces no astigmatism, where astigmatism leads to a reduced quality ofspatial vision i.e. to a reduced quality of the holographicreconstruction which is perceived by a viewer. It is therefore an objectof this implementation to achieve spatial multiplexing of VOWs incombination with 2D-encoding.

In this implementation, illumination with horizontal and vertical localspatial coherence is combined with a beam splitter that separates thelight into bundles of rays for the left eye VOW and for the right eyeVOW. Thereby the diffraction at the beam splitter is taken into account.The beam splitter may be a prism array, a second lens array (eg. astatic array, or a variable array eg. one as shown in FIG. 20) or abarrier mask.

An example of this implementation is shown in FIG. 25. FIG. 25 is aschematic drawing of a holographic display comprising light sources in a2D light source array, lenses in a 2D lens array, a SLM and abeamsplitter. The beamsplitter splits the rays leaving the SLM into twobundles each of which illuminates the virtual observer window for theleft eye (VOWL) and the virtual observer window for the right eye(VOWR), respectively. In this example, the number of light sources isone or more; the number of lenses equals the number of light sources.

In this example the beamsplitter is after the SLM. The positions ofbeamsplitter and SLM may also be swapped.

An example of this implementation is shown in FIG. 26, in plan view, inwhich a prism array is used as a beam splitter. The illuminationcomprises an n element 2D light-source array (LS1, LS2, . . . LSn) andan n element 2D lens array (L1, L2, . . . Ln), of which only two lightsources and two lenses are shown in FIG. 26. Each light source is imagedto the observer plane by its associated lens. The pitch of the lightsource array and the pitch of the lens array are such that alllight-source images coincide in the observer plane i.e. the plane whichcontains the two VOWs. In FIG. 26, the left eye VOW (VOWL) and the righteye VOW (VOWR) are not shown in the Figure, because they are locatedoutside the Figure, to the right of the Figure. An additional field lensmay be added. The pitch of the lens array is similar to the typical sizeof a sub-hologram in order to provide sufficient spatial coherence, i.e.the order of from one to several millimeters. The illumination ishorizontally and vertically spatially coherent within each lens, as thelight sources are small or point light sources and as a 2D lens array isused. The lens array may be refractive, diffractive or holographic.

In this example, the beamsplitter is a 1D array of vertical prisms. Thelight incident on one slope of a prism is deflected to the left eye VOW(to VOWL), the light incident on the other slope of the prism isdeflected to the right eye VOW (to VOWR). The rays that originate fromthe same LS and the same lens are also mutually coherent after passingthrough the beamsplitter. Hence, a 2D-encoding with vertical andhorizontal focusing and vertical and horizontal motion parallax ispossible.

The hologram is encoded on the SLM with 2D-encoding. The holograms forthe left and the right eye are interlaced column by column, i.e. thereare alternating columns encoded with left eye and right eye holograminformation. Preferably, under each prism there is column with a lefteye hologram information and a column with a right eye holograminformation. As an alternative, there may also be two or more columns ofa hologram under each slope of the prism, e.g. three columns for VOWLfollowed by three columns for VOWR, in succession. The pitch of the beamsplitter may be the same as, or an integer (such as two or three)multiple of, the pitch of the SLM, or the pitch of the beam splitter maybe slightly smaller than, or slightly smaller than an integer (such astwo or three) multiple of, the pitch of the SLM in order to accommodateperspective shortening.

Light from the columns with the left eye hologram reconstructs theobject for the left eye and illuminates the left eye VOW (VOWL); thelight from the columns with the right eye hologram reconstructs theobject for the right eye and illuminates the right eye VOW (VOWR). Thuseach eye perceives the appropriate reconstruction. If the pitch of theprism array is sufficiently small, the eye cannot resolve the prismstructure and the prism structure does not disturb the reconstructedimage. Each eye sees a reconstruction with full focusing and full motionparallax, and there is no astigmatism.

There will be diffraction at the beamsplitter as the beamsplitter isilluminated with coherent light. The beamsplitter may be regarded as adiffraction grating that generates multiple diffraction orders. Theslanted prism slopes have the effect of a blazed grating. At a blazedgrating, the maximum of the intensity is directed to a specificdiffraction order. At a prism array, one maximum of the intensity isdirected from one slope of the prisms to a diffraction order at theposition of VOWL, and another maximum of intensity is directed from theother slope of the prisms to another diffraction order at the positionof VOWR. To be more precise, the maxima in the intensities of theenveloping sinc-squared functions are shifted to these positions,whereas the diffraction orders are at fixed positions. The prism arraygenerates one intensity enveloping sinc-squared function maximum at theposition of VOWL and another intensity enveloping sinc-squared functionmaximum at the position of VOWR. The intensity of other diffractionorders will be small (i.e. the sinc squared intensity function maximumis narrow) and will not lead to a disturbing crosstalk as the fillfactor of the prism array is large, e.g. close to 100%.

As will be obvious to one skilled in the art, by using a more complexarray of prisms (eg. two types of prism with the same apex angles butdifferent degrees of asymmetry, disposed adjacent each other, insuccession) one may generate more VOWs, in order to provide VOWs for twoobservers, or for more than two observers. However, the observers cannotbe tracked individually with a static array of prisms.

In a further example, one may use more than one light source per lens.Additional light sources per lens can be used to generate additionalVOWs for additional observers. This is described in WO 2004/044659(US2006/0055994), for the case of one lens and m light sources for mobservers. In this further example, m light sources per lens and twofoldspatial multiplexing are used to generate m left VOWs and m right VOWsfor m observers. The m light sources per lens are in m-to-onecorrespondence with each lens, where m is a whole number.

Here is an example of this implementation. A 20 inch screen diagonal isused, with the following parameters: observer distance 2 m, pixel pitch69 μm in the vertical by 207 μm in the horizontal, Burckhardt encodingis used, and the optical wavelength is 633 nm. The Burckhardt encodingis in the vertical direction with a subpixel pitch of 69 μm and a VOWheight of 6 mm (vertical period). Neglecting the perspective shortening,the pitch of the array of vertical prisms is 414 μm, i.e. there are twocolumns of the SLM under each full prism. The horizontal period in theobserver plane is therefore 3 mm. This is also the width of the VOW.This width is smaller than optimal for an eye pupil of ca. 4 mm indiameter. In a further but similar example, if the SLM has a smallerpitch of 50 μm the VOW would have a width of 25 mm.

If a human adult has an eye separation of 65 mm (as is typical), theprisms have to deflect the light by ±32.5 mm where the light intersectsthe plane containing the VOWs. To be more precise, the intensityenveloping sinc-squared function maxima have to be deflected by ±32.5mm. This corresponds to an angle of ±0.93° for 2 m observer distance.The appropriate prism angle is ±1.86° for a prism refractive indexn=1.5. The prism angle is defined as the angle between the substrate andthe sloping side of a prism.

For a horizontal period in the observer plane of 3 mm, the other eye isat a distance of about 21 diffraction orders (i.e. 65 mm divided by 3mm). The crosstalk in VOWL and in VOWR caused by higher diffractionorders related to the other VOW is therefore negligible.

In order to implement tracking, a simple way of tracking is light-sourcetracking, i.e. adapting the light-source position. If SLM and prismarray are not in the same plane, there will be a disturbing relativelateral offset between the SLM pixels and the prisms, caused by theparallax. This may lead to disturbing crosstalk. The pixels of the 20inch screen diagonal example above may have a fill factor of 70% in thedirection perpendicular to the axes described by the peak of each of theprisms, i.e. the pixel dimensions are 145 μm active area and 31 μminactive margin on each side. If the structured area of the prism arrayis directed towards the SLM, the separation between prism array and SLMmay be ca. 1 mm. The horizontal tracking range without crosstalk wouldbe ±31 μm/1 mm*2 m=±62 mm. The tracking range would be larger if a smallcrosstalk were tolerated. This tracking range is not large but it issufficient to permit some tracking to take place, so that the viewerwill be less constrained as to where to position his/her eyes.

The parallax between SLM and prism array can be avoided, preferably byintegration of the prism array in or directly on the SLM (as arefractive, diffractive, or holographic prism array). This would be aspecialized component for a product. An alternative is lateralmechanical movement of the prism array, though this is not preferred asmoving mechanical parts would complicate the apparatus.

Another critical issue is the fixed separation of the VOWs which isgiven by the prism angle. This may lead to complications for observerswith non-standard eye separation or for z-tracking. As a solution, anassembly including encapsulated liquid-crystal domains may be used, suchas that shown in FIG. 21. An electric field may then control therefractive index and hence the deflection angle. This solution may beincorporated with a prism array, so as to give a variable deflection anda fixed deflection, respectively, in succession. In an alternativesolution, the structured side of the prism array might be covered by aliquid-crystal layer. An electric field might then control therefractive index and hence the deflection angle. A variable deflectionassembly is not necessary if the VOWs have such a large width that thereis sufficient tolerance for observers with different eye separations andfor z-tracking.

A more complex solution would be to use controllable prism arrays, e.g.e-wetting prism arrays (as shown in FIG. 27) or prisms filled withliquid crystals (as shown in FIG. 21). In FIG. 27, the layer with theprism element 159 comprises electrodes 1517, 1518 and a cavity filledwith two separate liquids 1519, 1520. Each liquid fills a prism-shapedpart of the cavity. As an example, the liquids may be oil and water. Theslope of the interface between the liquids 1519, 1520 depends on thevoltage applied to the electrodes 1517, 1518. If the liquids havedifferent refractive indices the light beam will experience a deviationthat depends on the voltage applied to the electrodes 1517, 1518. Hencethe prism element 159 acts as a controllable beam steering element. Thisis an important feature for the applicant's approach toelectro-holography for implementations which require tracking of VOWs tothe observers' eyes. Patent applications DE 102007024237.0, DE102007024236.2 filed by the applicant, which are incorporated herein byreference, describe tracking of VOWs to the observers' eyes with prismelements.

Here is an example of the implementation for use in a compact hand-helddisplay. Seiko® Epson® Corporation of Japan has released monochromeEASLMs, such as the D4:L3D13U1.3 inch screen diagonal panel. An exampleis described using the D4:L3D13U LCD panel as the SLM. It has HDTVresolution (1920 by 1080 pixels), 15 μm pixel pitch and a panel area of28.8 mm by 16.2 mm. This panel is usually used for 2D image projectiondisplays.

The example is calculated for a wavelength of 633 nm and an observerdistance of 50 cm. Detour-phase encoding (Burckhardt encoding) is usedfor this amplitude-modulating SLM: three pixels are needed to encode onecomplex number. These three associated pixels are vertically arranged.If the prism-array beamsplitter is integrated in the SLM, the pitch ofthe prism array is 30 μm. If there is a separation between SLM and prismarray, the pitch of the prism array is slightly different to account forthe perspective shortening.

The height of a VOW is determined by the pitch of 3*15 μm=45 μm toencode one complex number and is 7.0 mm. The width of the VOW isdetermined by the 30 μm pitch of the prism array and is 10.6 mm. Bothvalues are larger than the eye pupil. Therefore, each eye can see aholographic reconstruction if the VOWs are located at the eyes. Theholographic reconstructions are from 2D-encoded holograms and hence arewithout the astigmatism inherent in 1D-encoding, described above. Thisensures high quality of spatial vision and high quality of depthimpression.

As the eye separation is 65 mm, the prisms have to deflect the light by±32.5 mm. To be more precise, the intensity maxima of the envelopingsinc-squared intensity functions have to be deflected by ±32.5 mm. Thiscorresponds to an angle of ±3.72° for 0.5 m observer distance. Theappropriate prism angle is ±7.440 for a refractive index n=1.5. Theprism angle is defined as the angle between substrate and the slopingside of a prism.

For a horizontal period in the observer plane of 10.6 mm the other eyeis at a distance of ca. 6 diffraction orders (i.e. 65 mm divided by 10.6mm). The crosstalk caused by higher diffraction orders is thereforenegligible as the prism array has a high fill factor i.e. close to 100%.

Here is an example of the implementation for use in a large display. Aholographic display may be designed using a phase-modulating SLM with apixel pitch of 50 μm and a screen diagonal of 20 inches. For applicationas a TV the diagonal might rather be approximately 40 inches. Theobserver distance for this design is 2 m and the wavelength is 633 nm.

Two phase-modulating pixels of the SLM are used to encode one complexnumber. These two associated pixels are vertically arranged and thecorresponding vertical pitch is 2*50 μm=100 μm. With a prism arrayintegrated in the SLM, the horizontal pitch of the prism array is also2*50 μm=100 μm as each prism comprises two slopes and each slope is forone column of the SLM. The resulting width and height of a VOW of 12.7mm is larger than the eye pupil. Therefore, each eye can see aholographic reconstruction if the VOWs are located at the eyes. Theholographic reconstructions are from 2D-encoded holograms and hence arewithout the astigmatism inherent in 1D-encoding. This ensures highquality of spatial vision and high quality of depth impression.

As the eye separation is 65 mm, the prisms have to deflect the light by±32.5 mm. To be more precise, the maxima in the intensity envelopingsinc-squared functions have to be deflected by ±32.5 mm. Thiscorresponds to an angle of ±0.93° for 2 m observer distance. Theappropriate prism angle is ±1.86° for a refractive index n=1.5. Theprism angle is defined as the angle between the substrate and thesloping side of a prism.

The above examples are for distances of the observer from the SLM of 50cm and 2 m. More generally, the implementation may be applied fordistances of the observer from the SLM of between 20 cm and 4 m. Thescreen diagonal may be between 1 cm (such as for a mobile phonesub-display) and 50 inches (such as for a large size television).

Laser Light Sources

RGB solid state laser light sources, e.g. based on GaInAs or GaInAsNmaterials, may be suitable light sources for the compact holographicdisplay because of their compactness and their high degree of lightdirectionality. Such sources include the RGB vertical cavity surfaceemitting lasers (VCSEL) manufactured by Novalux® Inc., CA, USA. Suchsources may be supplied as single lasers or as arrays of lasers,although each source can be used to generate multiple beams through theuse of diffractive optical elements. The beams may be passed downmultimode optical fibres as this may reduce the coherence level if thecoherence is too high for use in compact holographic displays withoutleading to unwanted artefacts such as laser speckle patterns. Arrays oflaser sources may be one dimensional or two dimensional.

OLED Materials

Infra red emitting OLED materials have been demonstrated. For example,Del Caño et al. have demonstrated electroluminescence from OLEDmaterials based on perylenediimide-doped tris(8-quinolinolato)aluminium, as reported in Applied Physics Letters vol. 88, 071117(2006). An electroluminescence wavelength of 805 nm was demonstrated.Near infra red emitting OLED materials were reported by Domercq et al.in J Phys Chem B vol. 108, 8647-8651 (2004).

The preparation of OLED materials on transparent substrates has beendemonstrated. For example, in U.S. Pat. No. 7,098,591 OLED materials areprepared on transparent indium tin oxide electrodes. The electrodes areprepared on a transparent substrate, which may be borosilicate glass.These components may be incorporated into an OLED device which has atransparent substrate. The indium tin oxide layer may be sputtered ontothe substrate using a radio frequency magnetron sputtering tool. Theindium tin oxide may be sputtered using a target comprising indium oxideand tin oxide. The indium tin oxide layer may have an opticaltransmission of about 85% in the visible range. The indium tin oxide maybe smooth so as to avoid the creation of locally enhanced electricfields that may degrade the performance of the OLED material. A rootmean square roughness of less than about 2 nm may be preferable. Afunctional organic layer or layers may be deposited on the patternedelectrode surface. The thickness of the organic layers is typicallybetween 2 nm and 200 nm. A conductive layer may be patterned onto theorganic layers, so as to form an anode and a cathode either side of theorganic layer. The device may be sealed with a glass layer, to protectthe active layers from the environment.

Outline Manufacturing Process

The following describes the outline of a process for manufacturing thedevice of FIG. 2, but many variations of this process will be obvious tothose skilled in the art.

In a process for manufacturing the device of FIG. 2, a transparentsubstrate is selected. Such a substrate may be a rigid substrate such asa sheet of borosilicate glass which is about 200 μm thick, or it may bea flexible substrate such as a polymer substrate, such as apolycarbonate, acrylic, polypropylene, polyurethane, polystyrene,polyvinyl chloride or the like substrate. Transparent electrodes areprepared on the glass, as described in the previous section. An infrared emitting OLED material is deposited on the glass, and electricalcontacts are fabricated on the opposite side to the transparentelectrodes, as described in the previous section, such that pixellatedOLED infra red light emission is possible. The glass substrate may haveindentations for the OLED pixel material. The IR-OLED material may beprinted, sprayed or solution-processed onto the substrate. Anencapsulation layer, also an electrical isolation layer, is thendeposited on the OLED pixel layer. Such an encapsulation layer may be aninorganic insulator layer such as silicon dioxide, silicon nitride, orsilicon carbide, or it may be a polymerizable layer such as an epoxy.Deposition could be performed by sputtering or by chemical vapourdeposition in the case of the inorganic insulator layer, or it could beby printing or coating in the case of a polymerizable layer. Theencapsulation layer, also an electrical isolation layer, may have athickness of several micrometres, or less than ten micrometres. Theencapsulation layer is then covered by the photosensitive layer of theOASLM. The photosensor layer is sensitive in the infra red, transparentin the visible, and may have a thickness of several micrometres. Suchoptical properties may be provided by a dye that is absorbing in theinfra red. The OASLM is then completed by depositing a liquid crystallayer which is housed between two electrically conducting layers. Theliquid crystal layer may be configured for amplitude modulation or forphase modulation, and typically has a thickness of several micrometres.An infra red filter layer is then deposited on the device. This may beof the form of a polymer film with infra red absorbing pigments withinit, or it may be an inorganic layer such as a sputtered or chemicalvapour deposition grown silicon dioxide film with infra red absorbingcomponents within it.

It may be necessary for the layers between the two OASLM devices to besufficiently thick so as to ensure that the electric fields present inone OASLM do not affect the performance of the other OASLM. The infrared filter layer may be thick enough to achieve this objective. However,if the infra red filter layer is of insufficient thickness, the layerthickness may be increased such as by bonding the OASLM device using anoptical adhesive to a sheet of glass of sufficient thickness, or bydepositing a further optically transparent layer such as an inorganiclayer or a polymer layer as described above. The OASLM devices musthowever not be too far apart so that optical diffraction effects leaddetrimentally to pixel cross talk. For example, if the pixel width is 10micrometres it is preferable that the OASLM layers should be less than100 micrometres apart. The LC layer in one OASLM is configured toperform amplitude modulation; the LC layer in the other OASLM isconfigured to perform phase modulation.

The remainder of the device may be prepared in manner outlined above foreach of the OASLM and OLED layers. Alternatively, the remainder of thedevice may be prepared as a single unit which is then bonded onto thefirst part of the device, using for example a glass layer which ispresent for example to ensure sufficient separation between the OASLMlayers that the electric fields present in each OASLM do not influencethe operation of the other OASLM. Where the remainder of the device isprepared by depositing further material on the first part of the device,this may have the advantage that precision alignment of the pixels ofthe second OLED layer with the pixels of the first OLED layer isfacilitated.

Instead of having a separation layer with sufficient thickness next tothe OASLM it is also possible to use a thin separation layer that iscoated with a conducting transparent electrode, e.g. indium tin oxide.This electrode acts as a common electrode of the two liquid crystallayers. Furthermore, as a conducting electrode it is an equipotentialsurface. Therefore, it shields electric fields and prevents leakage ofelectric fields from one OASLM to the other OASLM.

An example of a device structure which may be fabricated using the aboveprocedures, or similar procedures, is given in FIG. 9. In use, thedevice structure 910 in FIG. 9 is illuminated by sufficiently coherentvisible radiation from the face 909 so that a viewer at point 911, whichis not shown at a distance from the device which is to scale withrespect to the device, may view a three dimensional image. The layers inthe device from 90 through to 908 are not necessarily to scale withrespect to each other. Layer 90 is a substrate layer, such as a glasslayer. Layer 91 is an OLED backplane layer, which provides electricalpower to the OLEDs, and may be wholly or partially transparent. Layer 92is an array of infra red emitting OLEDs. Layer 93 is a Bragg filterholographic element for at least partial infra red light collimation.Layer 93 may be omitted in some implementations. Layer 94 is anelectrical isolation layer. Layer 95 is an OASLM photosensor andelectrode layer. Layer 96 is a liquid crystal layer for amplitudemodulation of the visible light beams. Layer 97 is a separation layer,especially a thin separation layer. Layer 98 is a transparent electrodelayer. Layer 99 is a linear polarizer layer. Layer 900 is an infra redfilter layer which transmits visible light but which blocks infra redlight from the OLED arrays 92 and 906. Layer 901 is a liquid crystallayer for phase modulation of the visible light beams. Layer 902 is aseparation layer, especially a thin separation layer. Layer 903 is anOASLM photosensor and electrode layer. Layer 904 is an electricalisolation layer. Layer 905 is a Bragg filter holographic element for atleast partial infra red light collimation. Layer 905 may be omitted insome implementations. Layer 906 is an array of infra red emitting OLEDs.Layer 907 is an OLED backplane layer, which provides electrical power tothe OLEDs, and may be wholly or partially transparent. Layer 908 is aplane of covering material, such as glass. In manufacture, the device910 may be fabricated by starting with substrate layer 90 and depositingeach layer in turn until the final layer 908 is added. Such a procedurehas the advantage of facilitating that the layers of the structure maybe aligned in fabrication to high accuracy. Alternatively, the layersmay be fabricated in two or more parts and bonded together with asufficient degree of alignment.

For the fabrication of devices, it is very important that unwantedbirefringence, such as unwanted stress-induced birefringence, be kept toa minimum. Stress-induced birefringence causes linear or circularpolarization states of light to change into elliptical polarizationstates of light. The presence of elliptical polarization states of lightin the device where ideally linear or circular polarization states oflight would be present will reduce contrast and colour fidelity, andwill therefore degrade device performance.

Implementations

It will be appreciated by those skilled in the art that for the OASLMsin the embodiments above, a photosensitive layer which is transparent inthe visible range, but which absorbs in the infra red, is required. Inan alternative implementation, the photosensitive layer could bepatterned so as to have transparent gaps which transmit visible lightsuch as the red, green and blue beams, and non-transparent areas whichare sensitive to light from OLEDs. In this case, the photosensitivematerial need not be transparent to visible light. In addition, thewrite beams need not be infra red light. In one implementation, thewrite beams could be generated by a non-primary display colour such asby yellow light emitting OLEDs. The filter in between the two OASLMswould therefore need to have strong optical absorption in the yellow, soas to block yellow light, but still to have sufficient transmission atother optical wavelengths for the purpose of producing a functioningoptical display. In another implementation, the write beams could begenerated by ultra violet emitting OLEDs. The filter in between the twoOASLMs would therefore need to have strong optical absorption in theultra violet, so as to block ultra violet light, but still to havesufficient transmission at visible optical wavelengths for the purposeof producing a functioning optical display. Ultra violet emitting OLEDmaterials have been reported by Qiu et al. Applied Physics Letters 79,2276 (2001), and by Wong et al. Org. Lett. 7 (23), 5131 (2005). Inaddition, while the use of OLED materials has been emphasized, the useof other light emitting diode materials, or other display technologiessuch as Surface-conduction Electron-emitter Display (SED) technology ispossible.

While the implementations disclosed herein have emphasized thesuccessive encoding of amplitude and phase in the spatial lightmodulators, it will be appreciated by those skilled in the art that anysuccessive weighted encoding of two non-identical combinations ofamplitude and phase, that is two combinations which are not related bybeing equal through multiplication by any real number, but not by anycomplex number (excluding the real numbers), may be used in principle toencode a hologram pixel. The reason is that the vector space of thepossible holographic encodings of a pixel is spanned in the vector spacesense by any two non-identical combinations of amplitude and phase, thatis any two combinations which are not related by being equal throughmultiplication by any real number, but not by any complex number(excluding the real numbers).

In the Figures herein, the relative dimensions shown are not necessarilyto scale.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scope ofthis invention, and it should be understood that this invention is notto be unduly limited to the illustrative embodiments and implementationsset forth herein.

APPENDIX I Technical Primer

The following section is meant as a primer to several key techniquesused in some of the systems that implement the present invention.

In conventional holography, the observer can see a holographicreconstruction of an object (which could be a changing scene); hisdistance from the hologram is not however relevant. The reconstructionis, in one typical optical arrangement, at or near the image plane ofthe light source illuminating the hologram and hence is at the Fourierplane of the hologram. Therefore, the reconstruction has the samefar-field light distribution of the real world object that isreconstructed.

One early system (described in WO 2004/044659 and US 2006/0055994)defines a very different arrangement in which the reconstructed objectis not at or near the Fourier plane of the hologram at all. Instead, avirtual observer window zone is at the Fourier plane of the hologram;the observer positions his eyes at this location and only then can acorrect reconstruction be seen. The hologram is encoded on a LCD (orother kind of spatial light modulator) and illuminated so that thevirtual observer window becomes the Fourier transform of the hologram(hence it is a Fourier transform that is imaged directly onto the eyes);the reconstructed object is then the Fresnel transform of the hologramsince it is not in the focus plane of the lens. It is instead defined bya near-field light distribution (modelled using spherical wavefronts, asopposed to the planar wavefronts of a far field distribution). Thisreconstruction can appear anywhere between the virtual observer window(which is, as noted above, in the Fourier plane of the hologram) and theLCD or even behind the LCD as a virtual object.

There are several consequences to this approach. First, the fundamentallimitation facing designers of holographic video systems is the pixelpitch of the LCD (or other kind of light modulator). The goal is toenable large holographic reconstructions using LCDs with pixel pitchesthat are commercially available at reasonable cost. But in the past thishas been impossible for the following reason. The periodicity intervalbetween adjacent diffraction orders in the Fourier plane is given byλD/p, where λ is the wavelength of the illuminating light, D is thedistance from the hologram to the Fourier plane and p is the pixel pitchof the LCD. But in conventional holographic displays, the reconstructedobject is in the Fourier plane. Hence, a reconstructed object has to bekept smaller than the periodicity interval; if it were larger, then itsedges would blur into a reconstruction from an adjacent diffractionorder. This leads to very small reconstructed objects—typically just afew cm across, even with costly, specialised small pitch displays. Butwith the present approach, the virtual observer window (which is, asnoted above, positioned to be in the Fourier plane of the hologram) needonly be as large as the eye pupil. As a consequence, even LCDs with amoderate pitch size can be used. And because the reconstructed objectcan entirely fill the frustum between the virtual observer window andthe hologram, it can be very large indeed, i.e. much larger than theperiodicity interval. Further, where an OASLM is used, then there is nopixelation, and hence no periodicity, so that the constraint of keepingthe virtual observer window smaller than a periodicity interval nolonger applies.

There is another advantage as well, deployed in one variant. Whencomputing a hologram, one starts with one's knowledge of thereconstructed object—e.g. you might have a 3D image file of a racingcar. That file will describe how the object should be seen from a numberof different viewing positions. In conventional holography, the hologramneeded to generate a reconstruction of the racing car is deriveddirectly from the 3D image file in a computationally intensive process.But the virtual observer window approach enables a different and morecomputationally efficient technique. Starting with one plane of thereconstructed object, we can compute the virtual observer window as thisis the Fresnel transform of the object. We then perform this for allobject planes, summing the results to produce a cumulative Fresneltransform; this defines the wave field across the virtual observerwindow. We then compute the hologram as the Fourier transform of thisvirtual observer window. As the virtual observer window contains all theinformation of the object, only the single-plane virtual observer windowhas to be transformed to the hologram and not the multi-plane object.This is particularly advantageous if there is not a singletransformation step from the virtual observer window to the hologram butan iterative transformation like the Iterative Fourier TransformationAlgorithm. Each iteration step comprises only a single Fouriertransformation of the virtual observer window instead of one for eachobject plane, resulting in significantly reduced computation effort.

Another interesting consequence of the virtual observer window approachis that all the information needed to reconstruct a given object pointis contained within a relatively small section of the hologram; thiscontrasts with conventional holograms in which information toreconstruct a given object point is distributed across the entirehologram. Because we need encode information into a substantiallysmaller section of the hologram, that means that the amount ofinformation we need to process and encode is far lower than for aconventional hologram. That in turn means that conventionalcomputational devices (e.g. a conventional digital signal processor(DSP) with cost and performance suitable for a mass market device) canbe used even for real time video holography.

There are some less than desirable consequences however. First, theviewing distance from the hologram is important—the hologram is encodedand illuminated in such a way that only when the eyes are positioned atthe Fourier plane of the hologram is the optimal reconstruction seen;whereas in normal holograms, the viewing distance is not important.There are however various techniques for reducing this Z sensitivity ordesigning around it, and in practice the Z sensitivity of theholographic reconstruction is usually not extreme.

Also, because the hologram is encoded and illuminated in such a way thatoptimal holographic reconstructions can only be seen from a precise andsmall viewing position (i.e. precisely defined Z, as noted above, butalso X and Y co-ordinates), eye tracking may be needed. As with Zsensitivity, various techniques for reducing the X, Y sensitivity ordesigning around it exist. For example, as pixel pitch decreases (as itwill with LCD manufacturing advances), the virtual observer window sizewill increase. Furthermore, more efficient encoding techniques (likeKinoform encoding) facilitate the use of a larger part of theperiodicity interval as virtual observer window and hence the increaseof the virtual observer window.

The above description has assumed that we are dealing with Fourierholograms. The virtual observer window is in the Fourier plane of thehologram, i.e. in the image plane of the light source. As an advantage,the undiffracted light is focused in the so-called DC-spot. Thetechnique can also be used for Fresnel holograms where the virtualobserver window is not in the image plane of the light source. However,care must be taken that the undiffracted light is not visible as adisturbing background. Another point to note is that the term transformshould be construed to include any mathematical or computationaltechnique that is equivalent to or approximates to a transform thatdescribes the propagation of light. Transforms are merely approximationsto physical processes more accurately defined by Maxwellian wavepropagation equations; Fresnel and Fourier transforms are second orderapproximations, but have the advantages that (a) because they arealgebraic as opposed to differential, they can be handled in acomputationally efficient manner and (ii) can be accurately implementedin optical systems.

Further details are given in US patent application 2006-0138711, US2006-0139710 and US 2006-0250671, the contents of which are incorporatedby reference.

APPENDIX II Glossary of Terms Used in the Description

Computer Generated Hologram

A computer generated video hologram CGH is a hologram that is calculatedfrom a scene. The CGH may comprise complex-valued numbers representingthe amplitude and phase of light waves that are needed to reconstructthe scene. The CGH may be calculated e.g. by coherent ray tracing, bysimulating the interference between the scene and a reference wave, orby Fourier or Fresnel transform.

Encoding

Encoding is the procedure in which a spatial light modulator (e.g. itsconstituent cells, or contiguous regions for a continuous SLM like anOASLM) are supplied with control values of the video hologram. Ingeneral, a hologram comprises of complex-valued numbers representingamplitude and phase.

Encoded Area

The encoded area is typically a spatially limited area of the videohologram where the hologram information of a single scene point isencoded. The spatial limitation may either be realized by an abrupttruncation or by a smooth transition achieved by Fourier transform of avirtual observer window to the video hologram.

Fourier Transform

The Fourier transform is used to calculate the propagation of light inthe far field of the spatial light modulator. The wave front isdescribed by plane waves.

Fourier Plane

The Fourier plane contains the Fourier transform of the lightdistribution at the spatial light modulator. Without any focusing lensthe Fourier plane is at infinity. The Fourier plane is equal to theplane containing the image of the light source if a focusing lens is inthe light path close to the spatial light modulator.

Fresnel Transform

The Fresnel transform is used to calculate the propagation of light inthe near field of the spatial light modulator. The wave front isdescribed by spherical waves. The phase factor of the light wavecomprises a term that depends quadratically on the lateral coordinate.

Frustum

A virtual frustum is constructed between a virtual observer window andthe SLM and is extended behind the SLM. The scene is reconstructedinside this frustum. The size of the reconstructed scene is limited bythis frustum and not by the periodicity interval of the SLM.

Imaging Optics

Imaging optics are one or more optical components such as a lens, alenticular array, or a microlens array used to form an image of a lightsource (or light sources). References herein to an absence of imagingoptics imply that no imaging optics are used to form an image of the oneor two SLMs as described herein at a plane situated between the Fourierplane and the one or two SLMs, in constructing the holographicreconstruction.

Light System

The light system may include either of a coherent light source like alaser or a partially coherent light source like a LED. The temporal andspatial coherence of the partially coherent light source has to besufficient to facilitate a good scene reconstruction, i.e. the spectralline width and the lateral extension of the emitting surface have to besufficiently small.

Virtual Observer Window (VOW)

The virtual observer window is a virtual window in the observer planethrough which the reconstructed 3D object can be seen. The VOW is theFourier transform of the hologram and is positioned within oneperiodicity interval in order to avoid multiple reconstructions of theobject being visible. The size of the VOW has to be at least the size ofan eye pupil. The VOW may be much smaller than the lateral range ofobserver movement if at least one VOW is positioned at the observer'seyes with an observer tracking system. This facilitates the use of a SLMwith moderate resolution and hence small periodicity interval. The VOWcan be imagined as a keyhole through which the reconstructed 3D objectcan be seen, either one VOW for each eye or one VOW for both eyestogether.

Periodicity Interval

The CGH is sampled if it is displayed on a SLM composed of individuallyaddressable cells. This sampling leads to a periodic repetition of thediffraction pattern. The periodicity interval is λD/p, where λ is thewavelength, D the distance from the hologram to the Fourier plane, and pthe pitch of the SLM cells. OASLMs however have no sampling and hencethere is no periodic repetition of the diffraction pattern; therepetitions are in effect suppressed.

Reconstruction

The illuminated spatial light modulator encoded with the hologramreconstructs the original light distribution. This light distributionwas used to calculate the hologram. Ideally, the observer would not beable to distinguish the reconstructed light distribution from theoriginal light distribution. In most holographic displays the lightdistribution of the scene is reconstructed. In our display, rather thelight distribution in the virtual observer window is reconstructed.

Scene

The scene that is to be reconstructed is a real or computer generatedthree-dimensional light distribution. As a special case, it may also bea two-dimensional light distribution. A scene can constitute differentfixed or moving objects arranged in a space.

Spatial Light Modulator (SLM)

A SLM is used to modulate the wave front of the incoming light. An idealSLM would be capable of representing arbitrary complex-valued numbers,i.e. of separately controlling the amplitude and the phase of a lightwave. However, a typical conventional SLM controls only one property,either amplitude or phase, with the undesirable side effect of alsoaffecting the other property.

1. A holographic display device comprising an organic light emitting diode array (OLED array) writing onto an optically addressable spatial light modulator (OASLM), the OLED array and the OASLM forming adjacent layers, in which the OLED array and the OASLM are physically attached to one another indirectly via an isolation layer, the isolation layer comprising an angular filter, the OASLM encoding a hologram and a holographic reconstruction being generated by the device when an array of read beams illuminates the OASLM and the OASLM is suitably controlled by the OLED array.
 2. The device of claim 1 in which the OLED array and the OASLM form facing, adjacent layers with no intermediary imaging optics between the OLED array and the OASLM.
 3. The device of claim 1 in which the OLED array and OASLM are in fixed, direct or indirect physical attachment with one another.
 4. The device of claim 1 in which the angular filter is a Bragg filter.
 5. The device of claim 1 in which the OLED array emits a non-primary colour display wavelength and the read-out wavelengths are one or more of RGB.
 6. The device of claim 1 in which the OLED array is infra red (IR) emitting and writes to an IR sensitive layer on the OASLM.
 7. The device of claim 1 in which the OLED array and OASLM layers are reflective and visible light is reflected from the OLED array and OASLM layers to an observer.
 8. The device of claim 1 in which the OLED array is made up of multiple, smaller tiled OLEDs.
 9. The device of claim 1 in which the OASLM contains liquid crystal material or in which the OASLM includes a photosensitive dye which acts as a photosensor layer.
 10. The device of claim 1 in which the display is illuminated with a backlight and micro-lens array and in which the micro-lens array provides localized coherence over a small region of the display, that region being the only part of the display that encodes information used in reconstructing a given point of the reconstructed object.
 11. The device of claim 1 in which the OASLM is a Freedericksz cell arrangement to give phase control.
 12. The device of claim 1 in which holographic reconstruction is visible through a virtual observer window and in which virtual observer windows can be tiled using spatial or time multiplexing.
 13. The device of claim 1 in which the display is operable to time sequentially re-encode a hologram on the hologram-bearing medium for the left and then the right eye of an observer.
 14. The device of claim 1 in which the display generates a holographic reconstruction for a single user to view.
 15. The device of claim 1 in which the display generates a 2D image that is in focus on a screen independent of the distance of the screen from the device in the optical far field without the need for any projection lenses.
 16. The device of claim 1 in which a holographic image is sent to each eye of an observer using a beamsplitter.
 17. The device of claim 1 in which a beam steering element is present for tracking VOWs, the beam steering element comprising liquid crystal domains inside an isotropic host material, where the interfaces between the domains and the matrix are prism-shaped, or the shape of sections of a sphere, or the shape of sections of a cylinder, and the orientation of the liquid crystals are controlled using externally applied electric fields so as to vary the local refractive or diffractive properties of the beam steering element.
 18. The device of claim 1 in which the OASLM, a light source and a lens array aligned with the light source, are all housed within a portable casing and in which the light source is magnified between 10 and 60 times by the lens array.
 19. A method of generating a holographic reconstruction comprising the step of using a display device as claimed in claim
 1. 20. The device of claim 1 in which a beam steering element is present for tracking virtual observer windows (VOWs), the beam steering element comprising controllable prism arrays with prism elements, the prism array especially being in the form of an electro-wetting prism array, a prism element comprising electrodes and a cavity filled with two separate liquids and an interface between the liquids, the slope of the interface between the liquids being electrically controllable by applying voltage to the electrodes. 